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 <h1><center>数据库基础</center></h1>
 **作者：行癫（盗版必究）**
 ------
 ## 一：数据库简介
 #### 1.数据库技术构成
 		数据库系统 DBS
 				数据库管理系统（DataBase Management System， DBMS）
 						SQL(RDS):   ORACLE、 MySQL、MariaDB、SQL-SERVER
 						NoSQL:   Redis、MongoDB、Memcache
 				数据库管理员（DBA）
 		SQL语言（Structured Query Language 即结构化查询语言）	
 				DDL语句 	数据库定义语言： 数据库、表、视图、索引、存储过程、函数， CREATE  ALTER  //开发人员
 				DML语句     数据库操纵语言： 插入数据INSERT、删除数据DELETE,DROP、更新数据UPDATE    //开发人员
 				DQL语句     数据库查询语言： 查询数据 SELECT
 				DCL语句     数据库控制语言： 例如控制用户的访问权限GRANT、REVOKE
 		数据访问技术
 				ODBC PHP    <.php>
 				JDBC JAVA    <.jsp>
 				settings.py python <.py>
 #### 2.数据库部署
 ##### 版本
 		mysql：8.0
 		mysql：5.7
 ##### 网址
 		www.mysql.com
 ##### yum安装
 		下载mysql的yum仓库
 ```shell
 [root@xingdian ~]# wget https://dev.mysql.com/get/mysql80-community-release-el7-7.noarch.rpm
 ```
 		安装mysql的yum仓库
 ```shell
 [root@xingdian ~]# rpm -ivh https://dev.mysql.com/get/mysql80-community-release-el7-7.noarch.rpm
 ```
 		修改安装版本：
 ```shell
 方法一：命令方式
 [root@xingdian ~]# yum repolist all | grep mysql 
 [root@xingdian ~]# yum -y install yum-utils 
 [root@xingdian ~]# yum-config-manager --enable mysql57-community 
 [root@xingdian ~]# yum-config-manager --disable mysql80-community
 方法二：修改文件
 [root@xingdian ~]# vim /etc/yum.repos.d/mysql-community.repo
 ```
 		安装mysql
 ```shell
 [root@xingdian ~]# yum -y install mysql mysql-server
 ```
 		查看初始密码
 ```shell
 [root@xingdian ~]# grep 'password' /var/log/mysqld.log 
 2019-07-13T15:14:31.176905Z 1 [Note] A temporary password is generated for root@localhost: k12zPB1r;2Ta
 ```
 		修改密码
 ```shell
 [root@xingdian ~]# mysqladmin -u root -p'k12zPB1r;2Ta' password 'QianFeng@123'
 密码：大小写有特殊字符数字
 ```
 ##### 编译安装
 		准备编译所需的环境
 ```shell
 [root@xingdian ~]# yum -y install ncurses ncurses-devel openssl-devel bison gcc gcc-c++ make cmake
 ```
 		准备编译所需的安装包
 ```shell
 [root@xingdian ~]# wget https://dev.mysql.com/get/Downloads/MySQL-5.7/mysql-boost-5.7.39.tar.gz
 [root@xingdian ~]# tar xf mysql-boost-5.7.39.tar.gz
 ```
 ![image-20220919132739384](https://xingdian-image.oss-cn-beijing.aliyuncs.com/xingdian-image/image-20220919132739384.png)
 ![image-20220919132806056](https://xingdian-image.oss-cn-beijing.aliyuncs.com/xingdian-image/image-20220919132806056.png)
 ![image-20220919132826328](https://xingdian-image.oss-cn-beijing.aliyuncs.com/xingdian-image/image-20220919132826328.png)
 ![](https://xingdian-image.oss-cn-beijing.aliyuncs.com/xingdian-image/image-20220919133151912.png)
 		清理系统残留并创建新用户【可选操作】
 ```shell
 删除：
 [root@xingdian ~]# userdel -r mysql
 [root@xingdian ~]# yum -y remove mariadb mariadb-libs mariadb-server mariadb-devel
 [root@xingdian ~]# rm -rf /etc/my*
 [root@xingdian ~]# rm -rf /var/lib/mysql
 [root@xingdian ~]# rm -rf /var/log/mysql*
 创建：
 [root@xingdian ~]# groupadd mysql
 [root@xingdian ~]# useradd -r -g mysql -s /bin/nolgin mysql
 ```
 		配置
 ```shell
 [root@mysql-5.7.26 ~]# cmake . \
 -DWITH_BOOST=boost_1_59_0/ \
 -DCMAKE_INSTALL_PREFIX=/usr/local/mysql \
 -DSYSCONFDIR=/etc \指定安装目录配置文件的位置,默认就是etc
 -DMYSQL_DATADIR=/usr/local/mysql/data \数据目录 错误日志文件
 -DINSTALL_MANDIR=/usr/share/man \ 帮助文档的目录
 -DMYSQL_TCP_PORT=3306 \ 默认端口号3306
 -DMYSQL_UNIX_ADDR=/tmp/mysql.sock \ 用来做网络通信,启动的时候才会产生
 -DDEFAULT_CHARSET=utf8 \默认字符集
 -DEXTRA_CHARSETS=all \
 -DDEFAULT_COLLATION=utf8_general_ci \
 -DWITH_READLINE=1 \可以上下翻历史命令
 -DWITH_SSL=system \
 -DWITH_EMBEDDED_SERVER=1 \ 嵌入式服务器
 -DENABLED_LOCAL_INFILE=1 \ 支持从本机导入
 -DWITH_INNOBASE_STORAGE_ENGINE=1   //默认存储引擎
 提示:boost也可以使用如下指令自动下载
 -DDOWNLOAD_BOOST=1
 模板：
 cmake . \
 -DWITH_BOOST=boost/boost_1_59_0/ \
 -DCMAKE_INSTALL_PREFIX=/usr/local/mysql \
 -DSYSCONFDIR=/etc \
 -DMYSQL_DATADIR=/usr/local/mysql/data \
 -DINSTALL_MANDIR=/usr/share/man \
 -DMYSQL_TCP_PORT=3306 \
 -DMYSQL_UNIX_ADDR=/tmp/mysql.sock \
 -DDEFAULT_CHARSET=utf8 \
 -DEXTRA_CHARSETS=all \
 -DDEFAULT_COLLATION=utf8_general_ci \
 -DWITH_READLINE=1 \
 -DWITH_SSL=system \
 -DWITH_EMBEDDED_SERVER=1 \
 -DENABLED_LOCAL_INFILE=1 \
 -DWITH_INNOBASE_STORAGE_ENGINE=1
 ```
 		编译和安装
 ```shell
 [root@mysql-5.7.26 ~]# make && make install
 ```
 		初始化
 ```shell
 [root@xingdian ~]# cd /usr/local/mysql               把这个删了就相当于卸载
 [root@xingdian ~]# mkdir mysql-files
 [root@xingdian ~]# chown -R mysql.mysql /usr/local/mysql
 [root@xingdian ~]# ./bin/mysqld --initialize --user=mysql --basedir=/usr/local/mysql --datadir=/usr/local/mysql/data
 初始化,只需要初始化一次
 ```
 		启动mysql
 ```shell
 [root@xingdian ~]# ./bin/mysqld_safe --user=mysql &  （后台运行）
 [root@xingdian ~]# ./bin/mysqladmin -u root -p'原密码' password 123
 ```
 		客户端测试
 ```shell
 [root@xingdian ~]# ./bin/mysql -u root -p '密码'
 ```
 		扩展
 ```shell
 设置环境变量；可以直接使用mysql相关命令：
 [root@xingdian ~]# echo "export PATH=$PATH:/usr/local/mysql/bin" >> /etc/profile
 [root@xingdian ~]# source /etc/profile
 设置开机启动，并用systemctl管理mysql服务：
 [root@xingdian ~]# cp  /usr/local/mysql/support-files/mysql.server /etc/init.d/mysqld
 [root@xingdian ~]# chkconfig mysqld on    //做开机启动
 [root@xingdian ~]# systemctl start mysqld
 [root@xingdian ~]# systemctl stop mysqld
 ```
 #### 3.mysql基础
 ##### 编译安装
 ```shell
 [root@47ed2bec974d mysql]# ls
 COPYING       README       bin   include  mysql-test  support-files
 COPYING-test  README-test  docs  lib      share
 ```
 		bin目录：用于放置一些可执行文件，如mysql、mysqld、mysqlbinlog等
 		include目录：用于放置一些头文件，如：mysql.h、mysql_ername.h等
 		lib目录：用于放置一系列库文件
 		share目录：用于存放字符集、语言等信息。
 ##### yum安装
 		/var/lib/mysql   存放数据文件
 		/usr/share/mysql  用于存放字符集、语言等信息
 ##### 配置文件基本参数
 ```shell
 [root@mysql1 mysql]# vim /etc/my.cnf
 [mysqld]
 basedir=/usr/local/mysql
 datadir=/usr/local/mysql/data
 log-error=/var/log/mysqld.log
 pid-file=/var/run/mysqld/mysqld.pid
 validate_password=off   添加后可设置弱密码强度
 lower_case_table_names=1  不区分大小写
 扩展：
 key_buffer = 384M  //key_buffer是用于索引块的缓冲区大小，增加它可得到更好处理的索引(对所有读和多重写)。索引被所有的线程共享，key_buffer的大小视内存大小而定
 table_open_cache  = 512  //MySQL每打开一个表，都会读入一些数据到table_open_cache缓存中，当MySQL在这个缓存中找不到相应信息时，才会去磁盘上读取。默认值64, 假定系统有200个并发连接，则需将此参数设置为200*N(N为每个连接所需的文件描述符数目)；当把table_open_cache设置为很大时，如果系统处理不了那么多文件描述符，那么就会出现客户端失效，连接不上
 max_allowed_packet = 4M  //接受的数据包大小；增加该变量的值十分安全，这是因为仅当需要时才会分配额外内存。例如，仅当你发出长查询或MySQLd必须返回大的结果行时MySQLd才会分配更多内存。 该变量之所以取较小默认值是一种预防措施，以捕获客户端和服务器之间的错误信息包，并确保不会因偶然使用大的信息包而导致内存溢出
 sort_buffer_size = 2M  //MySQL执行排序使用的缓冲大小。如果想要增加ORDER BY的速度，首先看是否可以让MySQL使用索引而不是额外的排序阶段。如果不能，可以尝试增加sort_buffer_size变量的大小
 read_buffer_size = 2M  //读查询操作所能使用的缓冲区大小。和sort_buffer_size一样，该参数对应的分配内存也是每连接独享。对表进行顺序扫描的请求将分配一个读入缓冲区，MySQL会为它分配一段内存缓冲区。 如果对表的顺序扫描请求非常频繁，并且你认为频繁扫描进行得太慢，可以通过增加该变量值以及内存缓冲区大小提高其性能
 thread_concurrency      = 8  //最大并发线程数，取值为服务器逻辑CPU数量×2
 max_connections = 1000 //MySQL的最大连接数，如果服务器的并发连接请求量比较大，建议调高此值，以增加并行连接数量，当然这建立在机器能支撑的情况下，因为如果连接数越多，介于MySQL会为每个连接提供连接缓冲区，就会开销越多的内存，所以要适当调整该值，不能盲目提高设值
 ```
 #### 4.数据库引擎
 		数据库存储引擎是数据库底层软件组织，数据库管理系统（DBMS）使用数据引擎进行创建、查询、更新和删除数据。不同的存储引擎提供不同的存储机制、索引技巧、锁定水平等功能，使用不同的存储引擎，还可以 获得特定的功能。现在许多不同的数据库管理系统都支持多种不同的数据引擎。MySQL的核心就是存储引擎
 ##### InnoDB存储引擎
 		InnoDB是事务型数据库的首选引擎，支持事务安全表（ACID），支持行锁定和外键;InnoDB是默认的MySQL引擎
 特点：
 		支持事务处理，支持外键，支持崩溃修复能力和并发控制。如果需要对事务的完整性要求比较高（比如银行），要求实现并发控制（比如售票），那选择InnoDB有很大的优势。如果需要频繁的更新、删除操作的数据库，也可以选择InnoDB，因为支持事务的提交和回滚
 ##### MyISAM存储引擎
 		MyISAM基于ISAM存储引擎，并对其进行扩展。它是在Web、数据仓储和其他应用环境下最常使用的存储引擎之一。MyISAM拥有较高的插入、查询速度，但不支持事务
 特点：
 		插入数据快，空间和内存使用比较低。如果表主要是用于插入新记录和读出记录，那么选择MyISAM能实现处理高效率。如果应用的完整性、并发性要求比较低，也可以使用
 ##### MEMORY存储引擎
 		MEMORY存储引擎将表中的数据存储到内存中，为查询和引用其他表数据提供快速访问
 特点：
 		所有的数据都在内存中，数据的处理速度快，但是安全性不高。如果需要很快的读写速度，对数据的安全性要求较低，可以选择MEMOEY。它对表的大小有要求，不能建立太大的表。所以，这类数据库只使用在相对较小的数据库表
 ##### 如何选择引擎
 		如果要提供提交、回滚、崩溃恢复能力的事物安全（ACID兼容）能力，并要求实现并发控制，InnoDB是一个好的选择；如果数据表主要用来插入和查询记录，则MyISAM引擎能提供较高的处理效率；如果只是临时存放数据，数据量不大，并且不需要较高的数据安全性，可以选择将数据保存在内存中的Memory引擎；MySQL中使用该引擎作为临时表，存放查询的中间结果；如果只有INSERT和SELECT操作，可以选择Archive，Archive支持高并发的插入操作，但是本身不是事务安全的。Archive非常适合存储归档数据，如记录日志信息可以使用Archive。
 		使用哪一种引擎需要灵活选择，一个数据库中多个表可以使用不同引擎以满足各种性能和实际需求，使用合适的存储引擎，将会提高整个数据库的性能
 ##### 存储引擎查看
 ```shell
 MySQL [(none)]> show engines;
 +--------------------+---------+----------------------------------------------------------------+-----
 | Engine             | Support | Comment               | Transactions | XA   | Savepoints |
 +--------------------+---------+----------------------------------------------------------------+-----
 | InnoDB       | DEFAULT | Supports transactions, row-level locking, and foreign keys  | YES    | YES  | YES  |
 | MRG_MYISAM   | YES     | Collection of identical MyISAM tables         | NO     | NO   | NO   |
 | MEMORY       | YES     | Hash based, stored in memory, useful for temporary tables    | NO  | NO   | NO   |
 | BLACKHOLE    | YES     | /dev/null storage engine (anything you write to it disappears) | NO | NO | NO  |
 | MyISAM       | YES     | MyISAM storage engine               | NO           | NO   | NO   |
 | CSV          | YES     | CSV storage engine        | NO           | NO   | NO     |
 | ARCHIVE      | YES     | Archive storage engine        | NO    | NO   | NO  |
 | PERFORMANCE_SCHEMA | YES    | Performance Schema  | NO      | NO   | NO     |
 | FEDERATED    | NO  | Federated MySQL storage engine       | NULL         | NULL | NULL       |
 ```
 		Support列的值表示某种引擎是否能使用：YES表示可以使用、NO表示不能使用、DEFAULT表示该引擎为当前默认的存储引擎
 #### 5.mysql事务
 		MySQL 事务主要用于处理操作量大，复杂度高的数据。比如说，在人员管理系统中，你删除一个人员，你即需要删除人员的基本资料，也要删除和该人员相关的信息，如信箱，文章等等，这样，这些数据库操作语句就构成一个事务！
 		在 MySQL 中只有使用了 Innodb 数据库引擎的数据库或表才支持事务
 		事务处理可以用来维护数据库的完整性，保证成批的 SQL 语句要么全部执行，要么全部不执行
 		事务用来管理 insert,update,delete 语句
 ##### 满足事务条件
 		原子性：一个事务（transaction）中的所有操作，要么全部完成，要么全部不完成，不会结束在中间某个环节。事务在执行过程中发生错误，会被回滚（Rollback）到事务开始前的状态，就像这个事务从来没有执行过一样
 		一致性：在事务开始之前和事务结束以后，数据库的完整性没有被破坏。这表示写入的资料必须完全符合所有的预设规则，这包含资料的精确度、串联性以及后续数据库可以自发性地完成预定的工作		
 		隔离性：数据库允许多个并发事务同时对其数据进行读写和修改的能力，隔离性可以防止多个事务并发执行时由于交叉执行而导致数据的不一致。事务隔离分为不同级别，包括读未提交（Read uncommitted）、读提交（read committed）、可重复读（repeatable read）和串行化（Serializable）
 		持久性：事务处理结束后，对数据的修改就是永久的，即便系统故障也不会丢失
 ##### 了解
 ```sshell
 MYSQL 事务处理主要有两种方法：
 1、用 BEGIN, ROLLBACK, COMMIT来实现
 • BEGIN 开始一个事务
 • ROLLBACK 事务回滚
 • COMMIT 事务确认
 2、直接用 SET 来改变 MySQL 的自动提交模式:
 • SET AUTOCOMMIT=0 禁止自动提交
 • SET AUTOCOMMIT=1 开启自动提交
 >show variables like 'autocommit';     //查看是否修改成功
 注意：在编写应用程序时，最好事务的控制权限交给开发人员
 ```
 #### 6.数据库锁
 【阅读了解】
 		锁是计算机协调多个进程或纯线程并发访问某一资源的机制。在数据库中，除传统的计算资源（CPU、RAM、I/O）的争用以外，数据也是一种供许多用户共享的资源。如何保证数据并发访问的一致性、有效性是所在有数据库必须解决的一个问题，锁冲突也是影响数据库并发访问性能的一个重要因素。从这个角度来说，锁对数据库而言显得尤其重要，也更加复杂；相对其他数据库而言，MySQL的锁机制比较简单，其最显著的特点是不同的存储引擎支持不同的锁机制
 		MyISAM在执行查询语句（SELECT）前，会自动给涉及的所有表加读锁，在执行更新操作（UPDATE、DELETE、INSERT等）前，会自动给涉及的表加写锁，这个过程并不需要用户干预，因此用户一般不需要直接用LOCK TABLE命令给MyISAM表显式加锁。给MyISAM表显示加锁，一般是为了一定程度模拟事务操作，实现对某一时间点多个表的一致性读取
 ##### 锁分类
 		表级锁：开销小，加锁快；不会出现死锁；锁定粒度大，发生锁冲突的概率最高，并发度最低
 		行级锁：开销大，加锁慢；会出现死锁；锁定粒度最小，发生锁冲突的概率最低，并发度也最高
 		页面锁：开销和加锁时间界于表锁和行锁之间；会出现死锁；锁定粒度界于表锁和行锁之间，并发度一般
 ##### 案例
 		有一个订单表orders，其中记录有订单的总金额total，同时还有一个订单明细表order_detail，其中记录有订单每一产品的金额小计subtotal，假设我们需要检查这两个表的金额合计是否相等，可能就需要执行如下两条SQL：
 ```shell
 MySQL [(none)]> SELECT SUM(total) FROM orders;   
 MySQL [(none)]> SELECT SUM(subtotal) FROM order_detail;  
 ```
 		这时，如果不先给这两个表加锁，就可能产生错误的结果，因为第一条语句执行过程中order_detail表可能已经发生了改变。因此，正确的方法应该是：
 ```shell
 MySQL [(none)]> LOCK tables orders read local,order_detail read local;
 MySQL [(none)]> SELECT SUM(total) FROM orders;
 MySQL [(none)]> SELECT SUM(subtotal) FROM order_detail;
 MySQL [(none)]> Unlock tables;
 ```
 		上面的例子在LOCK TABLES时加了‘local’选项，其作用就是在满足MyISAM表并发插入条件的情况下，允许其他用户在表尾插入记录
 		在用LOCKTABLES给表显式加表锁是时，必须同时取得所有涉及表的锁，并且MySQL支持锁升级；也就是说，在执行LOCK TABLES后，只能访问显式加锁的这些表，不能访问未加锁的表；同时，如果加的是读锁，那么只能执行查询操作，而不能执行更新操作。其实，在自动加锁的情况下也基本如此，MySQL问题一次获得SQL语句所需要的全部锁
 #### 7.外键
 【阅读了解】
 		在实际开发的项目中，一个健壮数据库中的数据一定有很好的参照完整性。例如学生档案和成绩单两张表，如果成绩单中有张三的成绩，学生档案中张三的档案却被删除了，这样就会产生垃圾数据或者错误数据。为了保证数据的完整性，将两张表之间的数据建立关系，因此就需要在成绩表中添加外键约束
 		外键是指引用另外一个表中的一列或多列数据，被引用的列应该具有主键约束或者唯一性约束。外键用来建立和加强两个表数据之间的连接
 ##### 外键的定义
 		外键是某个表中的一列，它包含在另一个表的主键中
 		外键也是索引的一种，是通过一张表中的一列指向另一张表中的主键，来对两张表进行关联
 		一张表可以有一个外键，也可以存在多个外键，与多张表进行关联
 ##### 外键的作用
 		外键的主要作用是保证数据的一致性和完整性，并且减少数据冗余
--- a/mysql-MD/第三章：数据库查询.md
+++ b/mysql-MD/第三章：数据库查询.md
@ -0,0 +1,975 @@
 <h1><center>数据库查询</center></h1>
 **作者：行癫（盗版必究）**
 ------
 ## 一：基本查询
 #### 1.简介
 		单表查询
 		简单查询
 		通过条件查询
 		查询排序
 		限制查询记录数
 		使用集合函数查询
 		分组查询
 		使用正则表达式查询
 #### 2.案例
 创建案例所需表：company.employee5
 ```shell
 	雇员编号	 id				        int
 	雇员姓名	 name			       varchar(30)
 	雇员性别	 sex				   enum
 	雇用时期	 hire_date			   date
 	职位		  post					varchar(50)
 	职位描述	 job_description	    varchar(100)
 	薪水		   salary				double(15,2)
 	办公室		  office			    int
 	部门编号	 dep_id				   int
 ```
 ```shell
 MySQL [(none)]> CREATE TABLE company.employee5(
          id int primary key AUTO_INCREMENT not null,
          name varchar(30) not null,
          sex enum('male','female') default 'male' not null,
          hire_date date not null,
          post varchar(50) not null,
          job_description varchar(100),
          salary double(15,2) not null,
          office int,
          dep_id int
          );
 ```
 插入模拟数据：
 ```shell
 MySQL [(none)]> insert into company.employee5(name,sex,hire_date,post,job_description,salary,office,dep_id) values 
    ('jack','male','20180202','instructor','teach',5000,501,100),
    ('tom','male','20180203','instructor','teach',5500,501,100),
    ('robin','male','20180202','instructor','teach',8000,501,100),
    ('alice','female','20180202','instructor','teach',7200,501,100),
    ('','male','20180202','hr','hrcc',600,502,101),
    ('harry','male','20180202','hr',NULL,6000,502,101),
    ('emma','female','20180206','sale','salecc',20000,503,102),
    ('christine','female','20180205','sale','salecc',2200,503,102),
    ('zhuzhu','male','20180205','sale',NULL,2200,503,102),
    ('gougou','male','20180205','sale','',2200,503,102);
 ```
 语法格式：
 		select   字段名称,字段名称2......    from  表名  [条件]
 ##### a.简单查询	
 ```shell
 MySQL [company]> select * from employee5;
 +----+-----------+--------+------------+------------+-----------------+----------+--------+--------+
 | id | name      | sex    | hire_date  | post       | job_description | salary   | office | dep_id |
 +----+-----------+--------+------------+------------+-----------------+----------+--------+--------+
 |  1 | jack      | male   | 2018-02-02 | instructor | teach           |  5000.00 |    501 |    100 |
 |  2 | tom       | male   | 2018-02-03 | instructor | teach           |  5500.00 |    501 |    100 |
 |  3 | robin     | male   | 2018-02-02 | instructor | teach           |  8000.00 |    501 |    100 |
 |  4 | alice     | female | 2018-02-02 | instructor | teach           |  7200.00 |    501 |    100 |
 |  5 |           | male   | 2018-02-02 | hr         | hrcc            |   600.00 |    502 |    101 |
 |  6 | harry     | male   | 2018-02-02 | hr         | NULL            |  6000.00 |    502 |    101 |
 |  7 | emma      | female | 2018-02-06 | sale       | salecc          | 20000.00 |    503 |    102 |
 |  8 | christine | female | 2018-02-05 | sale       | salecc          |  2200.00 |    503 |    102 |
 |  9 | zhuzhu    | male   | 2018-02-05 | sale       | NULL            |  2200.00 |    503 |    102 |
 | 10 | gougou    | male   | 2018-02-05 | sale       |                 |  2200.00 |    503 |    102 |
 +----+-----------+--------+------------+------------+-----------------+----------+--------+--------+
 10 rows in set (0.00 sec)
 MySQL [company]> select name, salary, dep_id from employee5 where id <=5;
 +-------+---------+--------+
 | name  | salary  | dep_id |
 +-------+---------+--------+
 | jack  | 5000.00 |    100 |
 | tom   | 5500.00 |    100 |
 | robin | 8000.00 |    100 |
 | alice | 7200.00 |    100 |
 |       |  600.00 |    101 |
 +-------+---------+--------+
 5 rows in set (0.00 sec)
 ```
 ##### b.避免重复
 		不能部分使用DISTINCT，通常仅用于某一字段
 ```shell
 MySQL [company]> SELECT post FROM employee5;
 +------------+
 | post       |
 +------------+
 | instructor |
 | instructor |
 | instructor |
 | instructor |
 | hr         |
 | hr         |
 | sale       |
 | sale       |
 | sale       |
 | sale       |
 +------------+
 10 rows in set (0.00 sec)
 MySQL [company]> SELECT distinct post  FROM employee5;
 +------------+
 | post       |
 +------------+
 | instructor |
 | hr         |
 | sale       |
 +------------+
 3 rows in set (0.00 sec)
 ```
 ##### c.四则运算查询
 ```shell
 MySQL [company]> SELECT name, salary, salary*14 FROM employee5;
 +-----------+----------+-----------+
 | name      | salary   | salary*14 |
 +-----------+----------+-----------+
 | jack      |  5000.00 |  70000.00 |
 | tom       |  5500.00 |  77000.00 |
 | robin     |  8000.00 | 112000.00 |
 | alice     |  7200.00 | 100800.00 |
 |           |   600.00 |   8400.00 |
 | harry     |  6000.00 |  84000.00 |
 | emma      | 20000.00 | 280000.00 |
 | christine |  2200.00 |  30800.00 |
 | zhuzhu    |  2200.00 |  30800.00 |
 | gougou    |  2200.00 |  30800.00 |
 +-----------+----------+-----------+
 10 rows in set (0.01 sec)
 MySQL [company]> SELECT name, salary, salary*14 AS Annual_salary FROM employee5;
 +-----------+----------+---------------+
 | name      | salary   | Annual_salary |
 +-----------+----------+---------------+
 | jack      |  5000.00 |      70000.00 |
 | tom       |  5500.00 |      77000.00 |
 | robin     |  8000.00 |     112000.00 |
 | alice     |  7200.00 |     100800.00 |
 |           |   600.00 |       8400.00 |
 | harry     |  6000.00 |      84000.00 |
 | emma      | 20000.00 |     280000.00 |
 | christine |  2200.00 |      30800.00 |
 | zhuzhu    |  2200.00 |      30800.00 |
 | gougou    |  2200.00 |      30800.00 |
 +-----------+----------+---------------+
 10 rows in set (0.00 sec)
 MySQL [company]> SELECT name, salary, salary*14 Annual_salary FROM employee5;
 +-----------+----------+---------------+
 | name      | salary   | Annual_salary |
 +-----------+----------+---------------+
 | jack      |  5000.00 |      70000.00 |
 | tom       |  5500.00 |      77000.00 |
 | robin     |  8000.00 |     112000.00 |
 | alice     |  7200.00 |     100800.00 |
 |           |   600.00 |       8400.00 |
 | harry     |  6000.00 |      84000.00 |
 | emma      | 20000.00 |     280000.00 |
 | christine |  2200.00 |      30800.00 |
 | zhuzhu    |  2200.00 |      30800.00 |
 | gougou    |  2200.00 |      30800.00 |
 +-----------+----------+---------------+
 10 rows in set (0.00 sec)
 ```
 ##### d.定义显示格式
 		CONCAT() 函数用于连接字符串
 ```shell
 MySQL [company]> SELECT concat(name, 's annual salary: ', salary*14)  AS Annual_salary FROM employee5;
 +------------------------------------+
 | Annual_salary                      |
 +------------------------------------+
 | jacks annual salary: 70000.00      |
 | toms annual salary: 77000.00       |
 | robins annual salary: 112000.00    |
 | alices annual salary: 100800.00    |
 | s annual salary: 8400.00           |
 | harrys annual salary: 84000.00     |
 | emmas annual salary: 280000.00     |
 | christines annual salary: 30800.00 |
 | zhuzhus annual salary: 30800.00    |
 | gougous annual salary: 30800.00    |
 +------------------------------------+
 10 rows in set (0.00 sec)
 ```
 ##### e.单条件查询
 ```shell
 MySQL [company]> SELECT name,post FROM employee5 WHERE post='hr';
 +-------+------+
 | name  | post |
 +-------+------+
 |       | hr   |
 | harry | hr   |
 +-------+------+
 2 rows in set (0.00 sec)
 ```
 ##### f.多条件查询
 ```shell
 MySQL [company]> SELECT name,salary FROM employee5 WHERE post='hr' AND salary>10000;
 Empty set (0.00 sec)
 MySQL [company]> select * from employee5 where salary>5000 and salary<10000  or dep_id=102;
 +----+-----------+--------+------------+------------+-----------------+----------+--------+--------+
 | id | name      | sex    | hire_date  | post       | job_description | salary   | office | dep_id |
 +----+-----------+--------+------------+------------+-----------------+----------+--------+--------+
 |  2 | tom       | male   | 2018-02-03 | instructor | teach           |  5500.00 |    501 |    100 |
 |  3 | robin     | male   | 2018-02-02 | instructor | teach           |  8000.00 |    501 |    100 |
 |  4 | alice     | female | 2018-02-02 | instructor | teach           |  7200.00 |    501 |    100 |
 |  6 | harry     | male   | 2018-02-02 | hr         | NULL            |  6000.00 |    502 |    101 |
 |  7 | emma      | female | 2018-02-06 | sale       | salecc          | 20000.00 |    503 |    102 |
 |  8 | christine | female | 2018-02-05 | sale       | salecc          |  2200.00 |    503 |    102 |
 |  9 | zhuzhu    | male   | 2018-02-05 | sale       | NULL            |  2200.00 |    503 |    102 |
 | 10 | gougou    | male   | 2018-02-05 | sale       |                 |  2200.00 |    503 |    102 |
 +----+-----------+--------+------------+------------+-----------------+----------+--------+--------+
 8 rows in set (0.00 sec)
 ```
 ##### g.关键字
 		BETWEEN AND
 ```shell
 MySQL [company]> SELECT name,salary FROM employee5 WHERE salary BETWEEN 5000 AND 15000;
 +-------+---------+
 | name  | salary  |
 +-------+---------+
 | jack  | 5000.00 |
 | tom   | 5500.00 |
 | robin | 8000.00 |
 | alice | 7200.00 |
 | harry | 6000.00 |
 +-------+---------+
 5 rows in set (0.00 sec)
 MySQL [company]> SELECT name,salary FROM employee5  WHERE salary NOT BETWEEN 5000 AND 15000;
 +-----------+----------+
 | name      | salary   |
 +-----------+----------+
 |           |   600.00 |
 | emma      | 20000.00 |
 | christine |  2200.00 |
 | zhuzhu    |  2200.00 |
 | gougou    |  2200.00 |
 +-----------+----------+
 5 rows in set (0.00 sec)
 ```
 		IS NULL		
 ```shell
 MySQL [company]> SELECT name,job_description FROM employee5 WHERE job_description IS NULL;
 +--------+-----------------+
 | name   | job_description |
 +--------+-----------------+
 | harry  | NULL            |
 | zhuzhu | NULL            |
 +--------+-----------------+
 2 rows in set (0.00 sec)
 MySQL [company]> SELECT name,job_description FROM employee5 WHERE job_description IS NOT NULL;
 +-----------+-----------------+
 | name      | job_description |
 +-----------+-----------------+
 | jack      | teach           |
 | tom       | teach           |
 | robin     | teach           |
 | alice     | teach           |
 |           | hrcc            |
 | emma      | salecc          |
 | christine | salecc          |
 | gougou    |                 |
 +-----------+-----------------+
 8 rows in set (0.00 sec)
 MySQL [company]> SELECT name,job_description FROM employee5  WHERE job_description='';
 +--------+-----------------+
 | name   | job_description |
 +--------+-----------------+
 | gougou |                 |
 +--------+-----------------+
 1 row in set (0.00 sec)
 ```
 注意：（NULL说明）
 		等价于没有任何值、是未知数
 		NULL与0、空字符串、空格都不同,NULL没有分配存储空间
 		对空值做加、减、乘、除等运算操作，结果仍为空
 		比较时使用关键字用“is null”和“is not null”
 		排序时比其他数据都小（索引默认是降序排列，小→大），所以NULL值总是排在最前
 IN集合查询	
 ```shell
 MySQL [company]> SELECT name, salary FROM employee5 WHERE salary=4000 OR salary=5000 OR salary=6000 OR salary=9000 ;
 +-------+---------+
 | name  | salary  |
 +-------+---------+
 | jack  | 5000.00 |
 | harry | 6000.00 |
 +-------+---------+
 2 rows in set (0.00 sec)
 MySQL [company]> SELECT name, salary FROM employee5 WHERE salary IN (4000,5000,6000,9000) ;
 +-------+---------+
 | name  | salary  |
 +-------+---------+
 | jack  | 5000.00 |
 | harry | 6000.00 |
 +-------+---------+
 2 rows in set (0.00 sec)
 MySQL [company]> SELECT name, salary FROM employee5 WHERE salary NOT IN (4000,5000,6000,9000) ;
 +-----------+----------+
 | name      | salary   |
 +-----------+----------+
 | tom       |  5500.00 |
 | robin     |  8000.00 |
 | alice     |  7200.00 |
 |           |   600.00 |
 | emma      | 20000.00 |
 | christine |  2200.00 |
 | zhuzhu    |  2200.00 |
 | gougou    |  2200.00 |
 +-----------+----------+
 8 rows in set (0.01 sec)
 ```
 ##### h.模糊查询
 		关键字LIKE
 		通配符%：所有字符
 		通配符_： 一个字符
 ```shell
 MySQL [company]> SELECT * FROM employee5 WHERE name LIKE 'al%';
 +----+-------+--------+------------+------------+-----------------+---------+--------+--------+
 | id | name  | sex    | hire_date  | post       | job_description | salary  | office | dep_id |
 +----+-------+--------+------------+------------+-----------------+---------+--------+--------+
 |  4 | alice | female | 2018-02-02 | instructor | teach           | 7200.00 |    501 |    100 |
 +----+-------+--------+------------+------------+-----------------+---------+--------+--------+
 1 row in set (0.00 sec)
 MySQL [company]> SELECT * FROM employee5 WHERE name LIKE 'al___';
 +----+-------+--------+------------+------------+-----------------+---------+--------+--------+
 | id | name  | sex    | hire_date  | post       | job_description | salary  | office | dep_id |
 +----+-------+--------+------------+------------+-----------------+---------+--------+--------+
 |  4 | alice | female | 2018-02-02 | instructor | teach           | 7200.00 |    501 |    100 |
 +----+-------+--------+------------+------------+-----------------+---------+--------+--------+
 1 row in set (0.00 sec)
 ```
 ##### i.排序查询		
 ```shell
 MySQL [company]> select name,salary from employee5 order by salary;
 +-----------+----------+
 | name      | salary   |
 +-----------+----------+
 |           |   600.00 |
 | christine |  2200.00 |
 | zhuzhu    |  2200.00 |
 | gougou    |  2200.00 |
 | jack      |  5000.00 |
 | tom       |  5500.00 |
 | harry     |  6000.00 |
 | alice     |  7200.00 |
 | robin     |  8000.00 |
 | emma      | 20000.00 |
 +-----------+----------+
 10 rows in set (0.01 sec)
 MySQL [company]> select name,salary from employee5 order by salary desc; 
 +-----------+----------+
 | name      | salary   |
 +-----------+----------+
 | emma      | 20000.00 |
 | robin     |  8000.00 |
 | alice     |  7200.00 |
 | harry     |  6000.00 |
 | tom       |  5500.00 |
 | jack      |  5000.00 |
 | christine |  2200.00 |
 | zhuzhu    |  2200.00 |
 | gougou    |  2200.00 |
 |           |   600.00 |
 +-----------+----------+
 MySQL [company]> select name,salary from employee5 order by salary desc limit 3;   //控制显示前3行
 +-------+----------+
 | name  | salary   |
 +-------+----------+
 | emma  | 20000.00 |
 | robin |  8000.00 |
 | alice |  7200.00 |
 +-------+----------+
 3 rows in set (0.00 sec)
 MySQL [company]> select name,salary from employee5 order by salary desc limit 1,3;  //从序号1开始显示三行的内容
 +-------+---------+
 | name  | salary  |
 +-------+---------+
 | robin | 8000.00 |
 | alice | 7200.00 |
 | harry | 6000.00 |
 +-------+---------+
 3 rows in set (0.00 sec)
 ```
 注意：
 		ascending    美音 /ə'sɛndɪŋ/   升序
 		descending  美音 /dɪ'sɛndɪŋ/  降序
 ##### j.集合函数查询
 		count：可以查看共有多少条记录
 ```shell
 MySQL [company]> select count(*)  from employee5;
 +----------+
 | count(*) |
 +----------+
 |       10 |
 +----------+
 1 row in set (0.00 sec)
 MySQL [company]> select count(name) from employee5;
 +-------------+
 | count(name) |
 +-------------+
 |          10 |
 +-------------+
 1 row in set (0.00 sec)
 ```
 		max：查看最大值
 ```shell
 MySQL [company]> select max(salary) from employee5;
 +-------------+
 | max(salary) |
 +-------------+
 |    20000.00 |
 +-------------+
 1 row in set (0.00 sec)
 ```
 		min：查看最小值
 ```shell
 MySQL [company]> select min(salary) from employee5;
 +-------------+
 | min(salary) |
 +-------------+
 |      600.00 |
 +-------------+
 1 row in set (0.00 sec)
 ```
 		avg：查看平均值
 ```shell
 MySQL [company]> select avg(salary) from employee5;
 +-------------+
 | avg(salary) |
 +-------------+
 | 5890.000000 |
 +-------------+
 1 row in set (0.00 sec)
 ```
 		sum：求和
 		sale这个部门的总工资
 ```shell
 MySQL [company]> select concat("Total Department Wages：",sum(salary)) from employee5 where post='sale';
 +-------------------------------------------------+
 | concat("Total Department Wages：",sum(salary))  |
 +-------------------------------------------------+
 | Total Department Wages：26600.00                |
 +-------------------------------------------------+
 1 row in set (0.00 sec)
 ```
 		获取薪水最高的这个人的详细信息
 ```shell
 MySQL [company]> select * from employee5 where salary = (select max(salary) from employee5);
 +----+------+--------+------------+------+-----------------+----------+--------+--------+
 | id | name | sex    | hire_date  | post | job_description | salary   | office | dep_id |
 +----+------+--------+------------+------+-----------------+----------+--------+--------+
 |  7 | emma | female | 2018-02-06 | sale | salecc          | 20000.00 |    503 |    102 |
 +----+------+--------+------------+------+-----------------+----------+--------+--------+
 1 row in set (0.00 sec)
 ```
 ##### k.分组查询
 		GROUP BY和GROUP_CONCAT()函数一起使用
 		获取部门ID相同的员工并把名字拼接到一起
 ```shell
 MySQL [company]> SELECT dep_id,GROUP_CONCAT(name) FROM employee5 GROUP BY dep_id;
 +--------+------------------------------+
 | dep_id | GROUP_CONCAT(name)           |
 +--------+------------------------------+
 |    100 | jack,tom,robin,alice         |
 |    101 | ,harry                       |
 |    102 | emma,christine,zhuzhu,gougou |
 +--------+------------------------------+
 3 rows in set (0.01 sec)
 ```
 		GROUP BY和集合函数一起使用
 		获取部门最高薪资
 ```shell
 MySQL [company]> SELECT post,max(salary) FROM employee5 GROUP BY post;
 +------------+-------------+
 | post       | max(salary) |
 +------------+-------------+
 | hr         |     6000.00 |
 | instructor |     8000.00 |
 | sale       |    20000.00 |
 +------------+-------------+
 3 rows in set (0.00 sec)
 ```
 ##### l.正则查询
 ```shell
 以什么开头
 MySQL [company]> SELECT * FROM employee5 WHERE name REGEXP '^ali';
 +----+-------+--------+------------+------------+-----------------+---------+--------+--------+
 | id | name  | sex    | hire_date  | post       | job_description | salary  | office | dep_id |
 +----+-------+--------+------------+------------+-----------------+---------+--------+--------+
 |  4 | alice | female | 2018-02-02 | instructor | teach           | 7200.00 |    501 |    100 |
 +----+-------+--------+------------+------------+-----------------+---------+--------+--------+
 1 row in set (0.00 sec)
 以什么结尾
 MySQL [company]> SELECT * FROM employee5 WHERE name REGEXP 'ce$';
 +----+-------+--------+------------+------------+-----------------+---------+--------+--------+
 | id | name  | sex    | hire_date  | post       | job_description | salary  | office | dep_id |
 +----+-------+--------+------------+------------+-----------------+---------+--------+--------+
 |  4 | alice | female | 2018-02-02 | instructor | teach           | 7200.00 |    501 |    100 |
 +----+-------+--------+------------+------------+-----------------+---------+--------+--------+
 1 row in set (0.01 sec)
 连续出现n次
 MySQL [company]> SELECT * FROM employee5 WHERE name REGEXP 'm{2}';
 +----+------+--------+------------+------+-----------------+----------+--------+--------+
 | id | name | sex    | hire_date  | post | job_description | salary   | office | dep_id |
 +----+------+--------+------------+------+-----------------+----------+--------+--------+
 |  7 | emma | female | 2018-02-06 | sale | salecc          | 20000.00 |    503 |    102 |
 +----+------+--------+------------+------+-----------------+----------+--------+--------+
 1 row in set (0.00 sec)
 ```
 ## 二：多表联合查询
 【扩展了解】
 #### 1.数据准备
 ##### 表：company.employee6
 创建表：
 ```shell
 MySQL [company]> create table employee6( 
     emp_id int auto_increment primary key not null, 
     emp_name varchar(50), 
     age int, 
     dept_id int);
 Query OK, 0 rows affected (0.65 sec)
 ```
 查看表结构：
 ```shell
 MySQL [company]> desc employee6;
 +----------+-------------+------+-----+---------+----------------+
 | Field    | Type        | Null | Key | Default | Extra          |
 +----------+-------------+------+-----+---------+----------------+
 | emp_id   | int(11)     | NO   | PRI | NULL    | auto_increment |
 | emp_name | varchar(50) | YES  |     | NULL    |                |
 | age      | int(11)     | YES  |     | NULL    |                |
 | dept_id  | int(11)     | YES  |     | NULL    |                |
 +----------+-------------+------+-----+---------+----------------+
 4 rows in set (0.00 sec)
 ```
 插入模拟数据：
 ```shell
 MySQL [company]> insert into employee6(emp_name,age,dept_id) values
     ('',19,200),
     ('tom',26,201),
     ('jack',30,201),
     ('alice',24,202),
     ('robin',40,200),
     ('xingdian',16,200),
     ('natasha',28,204);
 ```
 查看数据：
 ```shell
 MySQL [company]> select * from employee6;
 +--------+----------+------+---------+
 | emp_id | emp_name | age  | dept_id |
 +--------+----------+------+---------+
 |      1 |          |   19 |     200 |
 |      2 | tom      |   26 |     201 |
 |      3 | jack     |   30 |     201 |
 |      4 | alice    |   24 |     202 |
 |      5 | robin    |   40 |     200 |
 |      6 | xingdian |   16 |     200 |
 |      7 | natasha  |   28 |     204 |
 +--------+----------+------+---------+
 7 rows in set (0.00 sec)
 ```
 ##### 表：company.department6
 创建表：
 ```shell
 MySQL [company]> create table department6(
     dept_id int,
     dept_name varchar(100)
     );
 Query OK, 0 rows affected (0.33 sec)
 ```
 查看表结构：
 ```shell
 MySQL [company]> desc department6;
 +-----------+--------------+------+-----+---------+-------+
 | Field     | Type         | Null | Key | Default | Extra |
 +-----------+--------------+------+-----+---------+-------+
 | dept_id   | int(11)      | YES  |     | NULL    |       |
 | dept_name | varchar(100) | YES  |     | NULL    |       |
 +-----------+--------------+------+-----+---------+-------+
 2 rows in set (0.00 sec)
 ```
 模拟插入数据：
 ```shell
 MySQL [company]> insert into department6 values
     (200,'hr'),
     (201,'it'),
     (202,'sale'),
     (203,'fd');
 ```
 查看数据：
 ```shell
 MySQL [company]> select * from department6;
 +---------+-----------+
 | dept_id | dept_name |
 +---------+-----------+
 |     200 | hr        |
 |     201 | it        |
 |     202 | sale      |
 |     203 | fd        |
 +---------+-----------+
 4 rows in set (0.01 sec)
 ```
 #### 2.多表的连接查询
 		交叉连接：生成笛卡尔积，它不使用任何匹配条件；交叉联接返回左表中的所有行，左表中的每一行与右表中的所有行组合
 		内连接：只连接匹配的行
 				外连接
 						左连接：会显示左边表内所有的值，不论在右边表内匹不匹配
 						右连接：会显示右边表内所有的值，不论在左边表内匹不匹配
 				全外连接：包含左、右两个表的全部行
 ##### 交叉连接
 ```shell
 MySQL [company]> select employee6.emp_name,employee6.age,employee6.dept_id,department6.dept_name from employee6,department6;
 +----------+------+---------+-----------+
 | emp_name | age  | dept_id | dept_name |
 +----------+------+---------+-----------+
 |          |   19 |     200 | hr        |
 |          |   19 |     200 | it        |
 |          |   19 |     200 | sale      |
 |          |   19 |     200 | fd        |
 | tom      |   26 |     201 | hr        |
 | tom      |   26 |     201 | it        |
 | tom      |   26 |     201 | sale      |
 | tom      |   26 |     201 | fd        |
 | jack     |   30 |     201 | hr        |
 | jack     |   30 |     201 | it        |
 | jack     |   30 |     201 | sale      |
 | jack     |   30 |     201 | fd        |
 | alice    |   24 |     202 | hr        |
 | alice    |   24 |     202 | it        |
 | alice    |   24 |     202 | sale      |
 | alice    |   24 |     202 | fd        |
 | robin    |   40 |     200 | hr        |
 | robin    |   40 |     200 | it        |
 | robin    |   40 |     200 | sale      |
 | robin    |   40 |     200 | fd        |
 | xingdian |   16 |     200 | hr        |
 | xingdian |   16 |     200 | it        |
 | xingdian |   16 |     200 | sale      |
 | xingdian |   16 |     200 | fd        |
 | natasha  |   28 |     204 | hr        |
 | natasha  |   28 |     204 | it        |
 | natasha  |   28 |     204 | sale      |
 | natasha  |   28 |     204 | fd        |
 +----------+------+---------+-----------+
 28 rows in set (0.00 sec)
 ```
 ##### 内连接
 		获取有部门的员工 (部门表中没有natasha所在的部门)
 ```shell
 MySQL [company]> select employee6.emp_name,employee6.age,employee6.dept_id,department6.dept_name from employee6,department6 where employee6.dept_id=department6.dept_id;
 +----------+------+---------+-----------+
 | emp_name | age  | dept_id | dept_name |
 +----------+------+---------+-----------+
 |          |   19 |     200 | hr        |
 | tom      |   26 |     201 | it        |
 | jack     |   30 |     201 | it        |
 | alice    |   24 |     202 | sale      |
 | robin    |   40 |     200 | hr        |
 | xingdian |   16 |     200 | hr        |
 +----------+------+---------+-----------+
 6 rows in set (0.00 sec)
 MySQL [company]> select employee6.emp_name,department6.dept_name from employee6 inner join department6 on employee6.dept_id=department6.dept_id;
 +----------+-----------+
 | emp_name | dept_name |
 +----------+-----------+
 |          | hr        |
 | tom      | it        |
 | jack     | it        |
 | alice    | sale      |
 | robin    | hr        |
 | xingdian | hr        |
 +----------+-----------+
 6 rows in set (0.01 sec)
 ```
 ##### 外连接
 语法：
 		SELECT 字段列表  FROM 表1 LEFT|RIGHT JOIN 表2  ON 表1.字段 = 表2.字段;
 注意：
 		先用谁谁就是左
 ###### 左连接 left join
 ```shell
 找出所有员工及所属的部门，包括没有部门的员工
 MySQL [company]> select emp_id,emp_name,dept_name from  employee6 left join department6 on employee6.dept_id = department6.dept_id;
 +--------+----------+-----------+
 | emp_id | emp_name | dept_name |
 +--------+----------+-----------+
 |      1 |          | hr        |
 |      5 | robin    | hr        |
 |      6 | xingdian | hr        |
 |      2 | tom      | it        |
 |      3 | jack     | it        |
 |      4 | alice    | sale      |
 |      7 | natasha  | NULL      |
 +--------+----------+-----------+
 7 rows in set (0.00 sec)
 ```
 ###### 右连接right join
 ```
 找出所有部门包含的员工，包括空部门
 MySQL [company]> select emp_id,emp_name,dept_name from  employee6 right join department6 on employee6.dept_id = department6.dept_id;
 +--------+----------+-----------+
 | emp_id | emp_name | dept_name |
 +--------+----------+-----------+
 |      1 |          | hr        |
 |      2 | tom      | it        |
 |      3 | jack     | it        |
 |      4 | alice    | sale      |
 |      5 | robin    | hr        |
 |      6 | xingdian | hr        |
 |   NULL | NULL     | fd        |
 +--------+----------+-----------+
 7 rows in set (0.00 sec)
 ```
 ##### 全外连接
 ```shell
 MySQL [company]> select * from employee6 full join department6;
 +--------+----------+------+---------+---------+-----------+
 | emp_id | emp_name | age  | dept_id | dept_id | dept_name |
 +--------+----------+------+---------+---------+-----------+
 |      1 |          |   19 |     200 |     200 | hr        |
 |      1 |          |   19 |     200 |     201 | it        |
 |      1 |          |   19 |     200 |     202 | sale      |
 |      1 |          |   19 |     200 |     203 | fd        |
 |      2 | tom      |   26 |     201 |     200 | hr        |
 |      2 | tom      |   26 |     201 |     201 | it        |
 |      2 | tom      |   26 |     201 |     202 | sale      |
 |      2 | tom      |   26 |     201 |     203 | fd        |
 |      3 | jack     |   30 |     201 |     200 | hr        |
 |      3 | jack     |   30 |     201 |     201 | it        |
 |      3 | jack     |   30 |     201 |     202 | sale      |
 |      3 | jack     |   30 |     201 |     203 | fd        |
 |      4 | alice    |   24 |     202 |     200 | hr        |
 |      4 | alice    |   24 |     202 |     201 | it        |
 |      4 | alice    |   24 |     202 |     202 | sale      |
 |      4 | alice    |   24 |     202 |     203 | fd        |
 |      5 | robin    |   40 |     200 |     200 | hr        |
 |      5 | robin    |   40 |     200 |     201 | it        |
 |      5 | robin    |   40 |     200 |     202 | sale      |
 |      5 | robin    |   40 |     200 |     203 | fd        |
 |      6 | xingdian |   16 |     200 |     200 | hr        |
 |      6 | xingdian |   16 |     200 |     201 | it        |
 |      6 | xingdian |   16 |     200 |     202 | sale      |
 |      6 | xingdian |   16 |     200 |     203 | fd        |
 |      7 | natasha  |   28 |     204 |     200 | hr        |
 |      7 | natasha  |   28 |     204 |     201 | it        |
 |      7 | natasha  |   28 |     204 |     202 | sale      |
 |      7 | natasha  |   28 |     204 |     203 | fd        |
 +--------+----------+------+---------+---------+-----------+
 28 rows in set (0.00 sec)
 ```
 #### 3.复合条件连接查询
 ##### 案例一
 		找出公司所有部门中年龄大于25岁的员工
 		以内连接的方式查询employee6和department6表，并且employee6表中的age字段值必须大于25
 ```shell
 MySQL [company]> select emp_id,emp_name,dept_name FROM employee6,department6 WHERE employee6.dept_id = department6.dept_id AND age > 25;
 +--------+----------+-----------+
 | emp_id | emp_name | dept_name |
 +--------+----------+-----------+
 |      5 | robin    | hr        |
 |      2 | tom      | it        |
 |      3 | jack     | it        |
 +--------+----------+-----------+
 3 rows in set (0.01 sec)
 ```
 ##### 案例二
 		以内连接的方式查询employee6和department6表，并且以age字段的升序方式显示
 ```shell
 MySQL [company]> select emp_id,emp_name,dept_name FROM employee6,department6 WHERE employee6.dept_id = department6.dept_id ORDER BY age asc;
 +--------+----------+-----------+
 | emp_id | emp_name | dept_name |
 +--------+----------+-----------+
 |      6 | xingdian | hr        |
 |      1 |          | hr        |
 |      4 | alice    | sale      |
 |      2 | tom      | it        |
 |      3 | jack     | it        |
 |      5 | robin    | hr        |
 +--------+----------+-----------+
 ```
 #### 4.子查询
 		子查询是将一个查询语句嵌套在另一个查询语句中
 		内层查询语句的查询结果，可以为外层查询语句提供查询条件
 		子查询中可以包含：IN、NOT IN等关键字；还可以包含比较运算符：= 、 !=、> 、<等
 ##### 案例一		
 		带IN关键字的子查询；查询employee表，但dept_id必须在department表中出现过
 ```shell
 MySQL [company]> select * from employee6 WHERE dept_id IN (select dept_id FROM department6);
 +--------+----------+------+---------+
 | emp_id | emp_name | age  | dept_id |
 +--------+----------+------+---------+
 |      1 |          |   19 |     200 |
 |      2 | tom      |   26 |     201 |
 |      3 | jack     |   30 |     201 |
 |      4 | alice    |   24 |     202 |
 |      5 | robin    |   40 |     200 |
 |      6 | xingdian |   16 |     200 |
 +--------+----------+------+---------+
 6 rows in set (0.00 sec)
 ```
 ##### 案例二
 		带比较运算符的子查询；查询年龄大于等于25岁员工所在部门（查询老龄化的部门）
 ```shell
 MySQL [company]> select dept_id,dept_name FROM department6 WHERE dept_id IN (SELECT DISTINCT dept_id FROM employee6 WHERE age >=25);
 +---------+-----------+
 | dept_id | dept_name |
 +---------+-----------+
 |     201 | it        |
 |     200 | hr        |
 +---------+-----------+
 2 rows in set (0.00 sec)
 ```
--- a/mysql-MD/第二章：数据库管理及数据类型.md
+++ b/mysql-MD/第二章：数据库管理及数据类型.md
@ -0,0 +1,861 @@
 <h1><center>数据库管理及数据类型</center></h1>
 **作者：行癫（盗版必究）**
 ------
 ## 一：数据类型
 #### 1.数值类型
 ##### 整数类型
 		整数类型：TINYINT SMALLINT MEDIUMINT INT BIGINT
 		作用：用于存储用户的年龄、游戏的Level、经验值等
 ![image-20220920124821420](https://xingdian-image.oss-cn-beijing.aliyuncs.com/xingdian-image/image-20220920124821420.png)
 ##### 浮点数类型
 		浮点数类型：FLOAT DOUBLE
 		作用：用于存储用户的身高、体重、薪水等
 ![image-20220920124919373](https://xingdian-image.oss-cn-beijing.aliyuncs.com/xingdian-image/image-20220920124919373.png)
 ```shell
 float(5,3)      5宽度  3精度
 注意：
 	宽度不算小数点
 	宽度-精度=点前
 案例：
 MySQL [(none)]> create database diandian;
 Query OK, 1 row affected (0.00 sec)
 MySQL [(none)]> use diandian
 Database changed
 MySQL [diandian]> create table t1(id float(6,2));
 Query OK, 0 rows affected (0.24 sec)
 MySQL [diandian]> insert into t1 values('2.22');
 ```
 ##### 定点数类型
 		定点数类型：DEC
 		定点数在MySQL内部以字符串形式存储，比浮点数更精确，适合用来表示货币等精度高的数据
 ##### 位类型
 		位类型：BIT
 		BIT(M)可以用来存放多位二进制数，M范围从1~64，如果不写默认为1位
 #### 2.字符串类型
 		CHAR系列  	CHAR  VARCHAR
 		TEXT系列   	TINYTEXT TEXT MEDIUMTEXT LONGTEXT
 		BLOB 系列	    TINYBLOB BLOB MEDIUMBLOB LONGBLOB 
 		BINARY系列 	BINARY VARBINARY 
 ![image-20220920132114919](https://xingdian-image.oss-cn-beijing.aliyuncs.com/xingdian-image/image-20220920132114919.png)
 ##### 枚举类型
 		枚举类型：枚举列可以把一些不重复的字符串存储成一个预定义的集合		
 ```shell
 mysql> create table enum_table( e ENUM('fish','apple','dog'));
 Query OK, 0 rows affected (0.35 sec)        
 mysql> insert into enum_table(e) values('fish');
 Query OK, 1 row affected (0.11 sec)
 mysql> select * from enum_table;
 +------+
 | e    |
 +------+
 | fish |
 +------+
 1 row in set (0.00 sec)
 mysql> insert into enum_table(e) values('nihao');
 ERROR 1265 (01000): Data truncated for column 'e' at row 1
 ```
 ##### 时间和日期类型
 		时间和日期类型：DATE TIME DATETIME TIMESTAMP YEAR 
 		作用：用于存储用户的注册时间，文章的发布时间，文章的更新时间，员工的入职时间等
 ![image-20220920132630856](https://xingdian-image.oss-cn-beijing.aliyuncs.com/xingdian-image/image-20220920132630856.png)
 ```shell
 mysql> create table t8 (
  id1 timestamp NOT NULL default CURRENT_TIMESTAMP,
  id2  datetime default NULL
 );
        timestamp 类型的列还有个特性：默认情况下，在 insert, update 数据时，timestamp 列会自动以当前时间（CURRENT_TIMESTAMP）填充/更新。“自动”的意思就是，你不去管它，MySQL 会替你去处理。
 mysql> insert into t8(id1) values('20180109000000');        
 mysql> select * from t8;
 +---------------------+------+
 | id1                 | d2   |
 +---------------------+------+
 | 2018-01-09 00:00:00 | NULL |
 +---------------------+------+
 1 row in set (0.00 sec)
 扩展： 
 select now();查看当前时间
 ```
 ## 二：表操作
 #### 1.案例
 表:：school.student1
 ```
 字段		    字段		        字段
 id             name	        sex		       age
 1              tom		    male	        23		 记录
 2              jack		    male	        21		 记录
 3              alice	    female	        19		  记录
 ```
 语法：
 ```shell
 create table 表名(自定义)(
 		字段名1  类型[(宽度) 约束条件],
 		字段名2  类型[(宽度) 约束条件],
 		字段名3  类型[(宽度) 约束条件]
 )[存储引擎 字符集];
 ==在同一张表中，字段名是不能相同
 ==宽度和约束条件可选
 ==字段名和类型是必须的
 ```
 创建库表：
 ```shell
 mysql> CREATE  DATABASE school;				        //创建数据库school
 mysql> use school;
 mysql> create table student1(
    id int,
    name varchar(50),                 
    sex enum('m','f'),
    age int
     );
 Query OK, 0 rows affected (0.03 sec)
 ```
 查看库：
 ```shell
 mysql> show tables;			                                    
 +------------------+
 | Tables_in_school |
 +------------------+
 | student1         |
 +------------------+
 1 row in set (0.00 sec)
 ```
 插入语法：
 ```
 insert into 表名(字段1,字段2...)  values(字段值列表...);
 ```
 插入数据：
 ```shell
 mysql> insert into student1(id,name,sex,age) values(1,'xingdia','m','26');
 ```
 查看表结构：
 ```shell
 mysql> desc student1;
 +-------+---------------+------+-----+---------+-------+
 | Field | Type          | Null | Key | Default | Extra |
 +-------+---------------+------+-----+---------+-------+
 | id    | int(11)       | YES  |     | NULL    |       |
 | name  | varchar(50)   | YES  |     | NULL    |       |
 | sex   | enum('m','f') | YES  |     | NULL    |       |
 | age   | int(11)       | YES  |     | NULL    |       |
 +-------+---------------+------+-----+---------+-------+
 4 rows in set (0.00 sec)
 ```
 查询数据：
 ```shell
 mysql> select id,name,sex,age from student1;	//查询表中所有字段的值
 Empty set (0.00 sec)
 mysql> select * from student1;		/查询表中所有字段的值
 Empty set (0.00 sec)
 mysql> select name,age from student1;		      //查询表中指定字段的值
 Empty set (0.00 sec)
 ```
 扩展插入：
 ```shell
 mysql> insert into student1 values (1,'xingdian','m',33),(2,'alice','m',20),(3,'jack','m',40);          //顺序插入
 Query OK, 3 rows affected (0.14 sec)
 Records: 3  Duplicates: 0  Warnings: 0
 mysql> insert into student1(name,age) values ('zhuzhu',10),('gougou',20);          //只向指定的字段插入值
 Query OK, 2 rows affected (0.12 sec)
 Records: 2  Duplicates: 0  Warnings: 0
 ```
 #### 2.案例
 表：school.student2
 ```shell
               字段名             数据类型
 编号             id                   int
 姓名			  name			      varchar(50)
 出生年份	     born_year	          year
 生日			  birthday		      date
 上课时间	     class_time	         time
 注册时间	     reg_time		     datetime
 ```
 创建表：
 ```shell
 mysql> create table student2(
    id int,
    name varchar(50),         
    born_year year,
    birthday date,
    class_time time,
    reg_time datetime 
    );
 ```
 插入数据：
 ```shell
 mysql> insert into student2 values(1,'tom',now(),now(),now(),now());
 mysql> insert into student2 values(2,'jack',1982,19821120,123000,20140415162545);
 ```
 表：school.student3
 ```shell
 id     id              int
 姓名	name	    varchar(50)
 性别	sex		    enum('male','female')
 爱好	hobby	    set('music','book','game','disc')
 ```
 创建表：
 ```shell
 mysql> create table student3(
     id int,
     name varchar(50),
     sex enum('male','female'),
     hobby set('music','book','game','disc')
     );
 ```
 查看表结构：
 ```shell
 mysql> desc student3;
 mysql> show create table student3\G
 ```
 插入数据：
 ```shell
 mysql> insert into student3 values (1,'tom','male','book,game');
 mysql> insert into student3 values (2,'jack','male','film');
 ```
 注意：
 		DESCRIBE查看表结构
 ```shell
 DESCRIBE 表名;
 DESC 表名;
 ```
 		查看表详细结构
 ```shell
 SHOW CREATE TABLE 表名;
 ```
 ## 三：表完整性约束
 #### 1.作用
 		用于保证数据的完整性和一致性
 #### 2.约束条件
 PRIMARY KEY (PK)		标识该字段为该表的主键，可以唯一的标识记录，不可以为空  UNIQUE + NOT NULL
 FOREIGN KEY (FK)		 标识该字段为该表的外键，实现表与表（父表主键/子表1外键/子表2外键）之间的关联
 NOT NULL						标识该字段不能为空
 UNIQUE KEY  (UK)		  标识该字段的值是唯一的，可以为空，一个表中可以有多个UNIQUE KEY
 AUTO_INCREMENT	    标识该字段的值自动增长（整数类型，而且为主键）
 DEFAULT						 为该字段设置默认值
 注意：
 		是否允许为空，默认NULL，可设置NOT NULL，字段不允许为空，必须赋值
 		字段是否有默认值，缺省的默认值是NULL，如果插入记录时不给字段赋值，此字段使用默认值
 ```shell
 MySQL [(none)]> sex enum('male','female') not null default 'male'
 MySQL [(none)]> age int unsigned NOT NULL default 20 		必须为正值（无符号） 不允许为空  默认是20
 ```
 		是否是key		主键 primary key		外键 forengn key
 #### 3.NOT NULL
 表：school.student4
 创建表：（注意前提需要有库）
 ```shell
 mysql> create table school.student4(
    id int not null,
    name varchar(50) not null,
    sex enum('m','f') default 'm' not null,
    age int unsigned default 18 not null,
    hobby set('music','disc','dance','book') default 'book,dance'
   );
 ```
 插入数据：（注意观察查询到的数据）
 ```shell
 MySQL [(none)]> insert into school.student4(id,name) values(2,'robin');
 Query OK, 1 row affected (0.08 sec)
 MySQL [(none)]> select * from school.student4;
 +----+-------+-----+-----+------------+
 | id | name  | sex | age | hobby      |
 +----+-------+-----+-----+------------+
 |  2 | robin | m   |  18 | dance,book |
 +----+-------+-----+-----+------------+
 1 row in set (0.00 sec)
 ```
 注意报错的原因：
 ```shell
 MySQL [(none)]> insert into school.student4 values(3,NULL,'m',40,'book');
 ERROR 1048 (23000): Column 'name' cannot be null
 ```
 #### 4.唯一约束
 作用：
 		MySQL索引的建立对于MySQL的高效运行是很重要的，索引可以大大提高MySQL的检索速度
 表：company.department1
 创建表：
 ```shell
 MySQL [(none)]> create database company;
 Query OK, 1 row affected (0.01 sec)
 MySQL [(none)]> CREATE TABLE company.department1 (dept_id INT,dept_name VARCHAR(30) UNIQUE,comment VARCHAR(50));
 Query OK, 0 rows affected (0.34 sec)
 ```
 查看表结构：
 ```shell
 MySQL [(none)]> desc company.department1;
 +-----------+-------------+------+-----+---------+-------+
 | Field     | Type        | Null | Key | Default | Extra |
 +-----------+-------------+------+-----+---------+-------+
 | dept_id   | int(11)     | YES  |     | NULL    |       |
 | dept_name | varchar(30) | YES  | UNI | NULL    |       |
 | comment   | varchar(50) | YES  |     | NULL    |       |
 +-----------+-------------+------+-----+---------+-------+
 3 rows in set (0.00 sec)
 ```
 数据插入：（注意查看插入数据时的提示）
 ```shell
 MySQL [(none)]> insert into company.department1 values ("1","xingdian","hr");
 Query OK, 1 row affected (0.04 sec)
 MySQL [(none)]> insert into company.department1 values ("1","xingdian","hr");
 ERROR 1062 (23000): Duplicate entry 'xingdian' for key 'dept_name'
 ```
 #### 5.主键约束
 注意：primary key 字段的值是不允许重复，且不允许不NULL（UNIQUE + NOT NULL）
 表：school.student6
 创建表：
 ```shell
 MySQL [(none)]> create table school.student6(
    ->     id int primary key not null auto_increment,
    ->     name varchar(50) not null,
    ->     sex enum('male','female') not null default 'male', 
    ->     age int not null default 18
    ->     );
 Query OK, 0 rows affected (0.47 sec)
 ```
 插入数据：
 ```shell
 MySQL [(none)]> insert into school.student6 values (1,'alice','female',22);
 Query OK, 1 row affected (0.18 sec)
 MySQL [(none)]> insert into school.student6(name,sex,age) values
    ->      ('jack','male',19),
    ->      ('tom','male',23);
 Query OK, 2 rows affected (0.14 sec)
 Records: 2  Duplicates: 0  Warnings: 0
 MySQL [(none)]> select * from school.student6;
 +----+-------+--------+-----+
 | id | name  | sex    | age |
 +----+-------+--------+-----+
 |  1 | alice | female |  22 |
 |  2 | jack  | male   |  19 |
 |  3 | tom   | male   |  23 |
 +----+-------+--------+-----+
 3 rows in set (0.00 sec)
 ```
 ## 四：修改表
 语法格式：
 修改表名
 	  ALTER TABLE 表名  RENAME 新表名;
 增加字段
 	  ALTER TABLE 表名
                    	  ADD 字段名  数据类型 [完整性约束条件…],
                          ADD 字段名  数据类型 [完整性约束条件…];
    ALTER TABLE 表名
 	                  	  ADD 字段名  数据类型 [完整性约束条件…]  AFTER 字段名;
 删除字段
 	ALTER TABLE 表名 DROP 字段名;
 修改字段
 	  ALTER TABLE 表名  MODIFY 字段名 数据类型 [完整性约束条件…];
 	  ALTER TABLE 表名  CHANGE 旧字段名 新字段名 旧数据类型 [完整性约束条件…];
 	  ALTER TABLE 表名  CHANGE 旧字段名 新字段名 新数据类型 [完整性约束条件…];
 #### 1.修改数据库引擎
 ```shell
 mysql> alter table service engine=innodb;     //engine=myisam|memory|....
 ```
 #### 2.添加字段
 ```shell
 mysql> create table student10 (id int);
 mysql> alter table student10 add name varchar(20) not null, add age int not null default 22;
 mysql> alter table student10 add stu_num int not null after name;   //添加name字段之后
 mysql> alter table student10 add sex enum('male','female') default 'male' first;  //添加到最前面
 ```
 #### 3.删除字段
 ```shell
 mysql> alter table student10 drop sex;
 ```
 #### 4.修改字段类型
 ```shell
 MySQL [school]> desc student10;
 +-------+---------+------+-----+---------+-------+
 | Field | Type    | Null | Key | Default | Extra |
 +-------+---------+------+-----+---------+-------+
 | id    | int(11) | YES  |     | NULL    |       |
 | age   | int(11) | YES  |     | NULL    |       |
 +-------+---------+------+-----+---------+-------+
 2 rows in set (0.01 sec)
 MySQL [school]>  alter table student10 modify age tinyint  not null ;  
 Query OK, 0 rows affected (0.04 sec)
 Records: 0  Duplicates: 0  Warnings: 0
 MySQL [school]> desc student10;
 +-------+------------+------+-----+---------+-------+
 | Field | Type       | Null | Key | Default | Extra |
 +-------+------------+------+-----+---------+-------+
 | id    | int(11)    | YES  |     | NULL    |       |
 | age   | tinyint(4) | NO   |     | NULL    |       |
 +-------+------------+------+-----+---------+-------+
 2 rows in set (0.00 sec)  
 MySQL [school]> alter table student10 modify id int not null primary key ;  ////修改字段类型、约束、主键
 Query OK, 0 rows affected (0.02 sec)
 Records: 0  Duplicates: 0  Warnings: 0
 MySQL [school]> desc student10;
 +-------+------------+------+-----+---------+-------+
 | Field | Type       | Null | Key | Default | Extra |
 +-------+------------+------+-----+---------+-------+
 | id    | int(11)    | NO   | PRI | NULL    |       |
 | age   | tinyint(4) | NO   |     | NULL    |       |
 +-------+------------+------+-----+---------+-------+
 ```
 #### 5.增加约束
 ```shell
 MySQL [school]> alter table student10 modify id int not null primary key ;
 Query OK, 0 rows affected (0.02 sec)
 Records: 0  Duplicates: 0  Warnings: 0
 MySQL [school]> desc student10;
 +-------+------------+------+-----+---------+-------+
 | Field | Type       | Null | Key | Default | Extra |
 +-------+------------+------+-----+---------+-------+
 | id    | int(11)    | NO   | PRI | NULL    |       |
 | age   | tinyint(4) | NO   |     | NULL    |       |
 +-------+------------+------+-----+---------+-------+
 2 rows in set (0.00 sec)
 MySQL [school]> alter table student10 modify id int not null primary key auto_increment; 
 ERROR 1068 (42000): Multiple primary key defined   //错误，该字段已经是primary key
 MySQL [school]> alter table student10 modify id int not null auto_increment;
 Query OK, 0 rows affected (0.04 sec)
 Records: 0  Duplicates: 0  Warnings: 0
 MySQL [school]> desc student10;
 +-------+------------+------+-----+---------+----------------+
 | Field | Type       | Null | Key | Default | Extra          |
 +-------+------------+------+-----+---------+----------------+
 | id    | int(11)    | NO   | PRI | NULL    | auto_increment |
 | age   | tinyint(4) | NO   |     | NULL    |                |
 +-------+------------+------+-----+---------+----------------+
 ```
 #### 6.增加主键
 ```shell
 MySQL [school]> desc student1;
 +-------+---------+------+-----+---------+-------+
 | Field | Type    | Null | Key | Default | Extra |
 +-------+---------+------+-----+---------+-------+
 | id    | int(11) | YES  |     | NULL    |       |
 | age   | int(11) | YES  |     | NULL    |       |
 | name  | char(1) | YES  |     | NULL    |       |
 +-------+---------+------+-----+---------+-------+
 3 rows in set (0.00 sec)
 MySQL [school]> alter table student1 add primary key(id);
 MySQL [school]> desc student1;
 +-------+---------+------+-----+---------+-------+
 | Field | Type    | Null | Key | Default | Extra |
 +-------+---------+------+-----+---------+-------+
 | id    | int(11) | NO   | PRI | NULL    |       |
 | age   | int(11) | YES  |     | NULL    |       |
 | name  | char(1) | YES  |     | NULL    |       |
 +-------+---------+------+-----+---------+-------+
 3 rows in set (0.00 sec)
 ```
 #### 7.修改主键和自增
 ```shell
 MySQL [school]> alter table student1 modify id int  auto_increment;
 Query OK, 0 rows affected (0.03 sec)
 Records: 0  Duplicates: 0  Warnings: 0
 MySQL [school]> desc student1;
 +-------+---------+------+-----+---------+----------------+
 | Field | Type    | Null | Key | Default | Extra          |
 +-------+---------+------+-----+---------+----------------+
 | id    | int(11) | NO   | PRI | NULL    | auto_increment |
 | age   | int(11) | YES  |     | NULL    |                |
 | name  | char(1) | YES  |     | NULL    |                |
 +-------+---------+------+-----+---------+----------------+
 ```
 #### 8.删除主键
 ```shell
 MySQL [school]> desc student10;
 +-------+------------+------+-----+---------+----------------+
 | Field | Type       | Null | Key | Default | Extra          |
 +-------+------------+------+-----+---------+----------------+
 | id    | int(11)    | NO   | PRI | NULL    | auto_increment |
 | age   | tinyint(4) | NO   |     | NULL    |                |
 +-------+------------+------+-----+---------+----------------+
 2 rows in set (0.00 sec)
 MySQL [school]> alter table student10 drop primary key;
 ERROR 1075 (42000): Incorrect table definition; there can be only one auto column and it must be defined as a key
 删除自增
 ySQL [school]>  alter table student10 modify id int not null;
 Query OK, 0 rows affected (0.04 sec)
 Records: 0  Duplicates: 0  Warnings: 0
 MySQL [school]> desc student10;
 +-------+------------+------+-----+---------+-------+
 | Field | Type       | Null | Key | Default | Extra |
 +-------+------------+------+-----+---------+-------+
 | id    | int(11)    | NO   | PRI | NULL    |       |
 | age   | tinyint(4) | NO   |     | NULL    |       |
 +-------+------------+------+-----+---------+-------+
 MySQL [school]> alter table student10 drop primary key;
 Query OK, 0 rows affected (0.03 sec)
 Records: 0  Duplicates: 0  Warnings: 0
 MySQL [school]> desc student10;
 +-------+------------+------+-----+---------+-------+
 | Field | Type       | Null | Key | Default | Extra |
 +-------+------------+------+-----+---------+-------+
 | id    | int(11)    | NO   |     | NULL    |       |
 | age   | tinyint(4) | NO   |     | NULL    |       |
 +-------+------------+------+-----+---------+-------+
 2 rows in set (0.00 sec)
 ```
 #### 9.复制表
 复制表结构＋记录 （key不会复制: 主键、外键和索引）复制表结构/记录+表结构，不会将Key复制
 ```shell
 mysql> create table new_service select * from service;
 ```
 只复制表结构
 ```shell
 mysql> create table new1_service select * from service where 1=2;   //条件为假，查不到任何记录
 ```
 可以复制主键,只复制表结构
 ```shell
 mysql> create table t4 like employees;
 ```
 #### 10.删除表
 ```shell
 mysql> DROP TABLE 表名;
 ```
 #### 11.修改数据表中字段的值
 语法：
 		Update 表名 set 列名=值where 条件
 ```shell
 mysql> update student set name='123' where id=1;
 ```
 删除某一行：
 		delete from 表名 where id=1
 ```shell
 mysql> delete from type where id=1;
 ```
 ## 五：库操作
 #### 1.简介
 		系统自带库的含义及作用
 ```shell
 MySQL [(none)]> show databases;
 +--------------------+
 | Database           |
 +--------------------+
 | information_schema |
 | mysql              |
 | performance_schema |
 | sys                |
 +--------------------+
 ```
 information_schema：虚拟库，主要存储了系统中的一些数据库对象的信息，例如用户表信息、列信息、权限信息、字符信息等
 performance_schema：主要存储数据库服务器的性能参数
 mysql：授权库，主要存储系统用户的权限信息
 sys：主要存储数据库服务器的性能参数
 注意：information_schema
 		SCHEMATA     存放的是系统中的库
 ```shell
 MySQL [information_schema]> select * from information_schema.SCHEMATA;
 +--------------+--------------------+----------------------------+------------------------+----------+
 | CATALOG_NAME | SCHEMA_NAME        | DEFAULT_CHARACTER_SET_NAME | DEFAULT_COLLATION_NAME | SQL_PATH |
 +--------------+--------------------+----------------------------+------------------------+----------+
 | def          | information_schema | utf8                       | utf8_general_ci        | NULL     |
 | def          | mysql              | latin1                     | latin1_swedish_ci      | NULL     |
 | def          | performance_schema | utf8                       | utf8_general_ci        | NULL     |
 | def          | school             | latin1                     | latin1_swedish_ci      | NULL     |
 | def          | sys                | utf8                       | utf8_general_ci        | NULL     |
 +--------------+--------------------+----------------------------+------------------------+----------+
 目录_名称
 实际库_名称
 默认_字符_设置_名称
 默认_分类_名称
 ```
 		TABLES   存储表名
 ```shell
 MySQL [information_schema]> select * from information_schema.TABLES\G
 *************************** 283. row ***************************
  TABLE_CATALOG: def
   TABLE_SCHEMA: xingdian
     TABLE_NAME: t1
     TABLE_TYPE: BASE TABLE
         ENGINE: InnoDB
        VERSION: 10
     ROW_FORMAT: Dynamic
     TABLE_ROWS: 1
 AVG_ROW_LENGTH: 16384
    DATA_LENGTH: 16384
 MAX_DATA_LENGTH: 0
   INDEX_LENGTH: 0
      DATA_FREE: 0
 AUTO_INCREMENT: NULL
    CREATE_TIME: 2022-09-22 08:18:38
    UPDATE_TIME: 2022-09-22 08:18:54
     CHECK_TIME: NULL
 TABLE_COLLATION: latin1_swedish_ci
       CHECKSUM: NULL
 CREATE_OPTIONS: 
  TABLE_COMMENT: 
 283 rows in set (0.02 sec)
 ```
 		COLUMNS     存储字段
 ```shell
 *************************** 3083. row ***************************
           TABLE_CATALOG: def
            TABLE_SCHEMA: xingdian
              TABLE_NAME: t1
             COLUMN_NAME: id
        ORDINAL_POSITION: 1
          COLUMN_DEFAULT: NULL
             IS_NULLABLE: YES
               DATA_TYPE: int
 CHARACTER_MAXIMUM_LENGTH: NULL
  CHARACTER_OCTET_LENGTH: NULL
       NUMERIC_PRECISION: 10
           NUMERIC_SCALE: 0
      DATETIME_PRECISION: NULL
      CHARACTER_SET_NAME: NULL
          COLLATION_NAME: NULL
             COLUMN_TYPE: int(11)
              COLUMN_KEY: 
                   EXTRA: 
              PRIVILEGES: select,insert,update,references
          COLUMN_COMMENT: 
   GENERATION_EXPRESSION: 
 3083 rows in set (0.03 sec)
 ```
 #### 2.创建库
 方案一：交互式操作
 ```shell
 mysql>  create database  xingdian;
 ```
 数据库命名规则：
 		区分大小写
 		唯一性
 		不能使用关键字如 create select
 		不能单独使用数字		
 方案二：非交互式
 ```shell
 mysql -u root -pQianFeng@123 -e "create database diandian"
 ```
 #### 3.查看数据库
 ```shell
 mysql> show databases;
 mysql> show create database xingdian;
 mysql> select database(); 查看当前所在的库
 ```
 #### 4.切换数据库
 ```shell
 mysql> use xingdian;
 ```
 #### 5.删除数据库
 ```shell
 mysql> DROP DATABASE 数据库名;
 ```
--- a/mysql-MD/第五章：权限管理及数据备份与恢复.md
+++ b/mysql-MD/第五章：权限管理及数据备份与恢复.md
--- a/mysql-MD/第四章：数据库日志管理.md
+++ b/mysql-MD/第四章：数据库日志管理.md
@ -0,0 +1,135 @@
 <h1><center>数据库日志管理</center></h1>
 **作者：行癫（盗版必究）**
 ------
 ## 一：日志管理
 <img src="https://xingdian-image.oss-cn-beijing.aliyuncs.com/xingdian-image/image-20220925214046253.png" alt="image-20220925214046253" style="zoom:50%;" />
 #### 1.日志分类
 		错误日志 ：启动，停止，关闭失败报错。rpm安装日志位置 /var/log/mysqld.log
 		通用查询日志：所有的查询都记下来
 		二进制日志：实现备份，增量备份。只记录改变数据，除了select都记
 		中继日志：读取主服务器的binlog，在本地回放。保持一致
 		slow log：慢查询日志，指导调优，定义某一个查询语句，定义超时时间，通过日志提供调优建议给开发人员
 		DDL log： 定义语句的日志
 #### 2.Error Log
 ```shell
 log-error=/var/log/mysqld.log
 ```
 #### 3.Binary Log
 ```
 log-bin=/var/log/mysql-bin/slave2
 server-id=2
 [root@slave2 ~]# mkdir /var/log/mysql-bin
 [root@slave2 ~]# chown mysql.mysql /var/log/mysql-bin/
 [root@slave2 ~]# systemctl restart mysqld
 ```
 注意：
 		需要提前开启
 查看binlog日志：
 ```shel
 [root@slave2 ~]# mysqlbinlog slave2-bin.000001     -v --base64-output=decode-rows
        时间点  ：  141126 14:04:49
        位置点  :     at  106
 注：
 1. 重启mysqld 会截断
 2. flush logs 会截断
 3. reset master 删除所有binlog    rm -rf /var/lib/mysql/*.000001
 4. 删除部分
 PURGE BINARY LOGS TO 'mysql-bin.010';
 PURGE BINARY LOGS BEFORE '2019-04-02 22:46:26';
 截取binlog
 all：
 # mysqlbinlog mysql.000002
 datetime：
 # mysqlbinlog mysql.000002 --start-datetime="2018-12-05 10:02:56"
 # mysqlbinlog mysql.000002 --stop-datetime="2018-12-05 11:02:54"
 # mysqlbinlog mysql.000002 --start-datetime="2018-12-05 10:02:56" --stop-datetime="2018-12-05 11:02:54" 
 position：
 # mysqlbinlog mysql.000002 --start-position=260
 # mysqlbinlog mysql.000002 --stop-position=260
 # mysqlbinlog mysql.000002 --start-position=260 --stop-position=930
 ```
 #### 4.Slow Query Log
 开启慢查询日志：
 ```shell
 [root@xingdian ~]# vim /etc/my.cnf
 slow_query_log=1
 slow_query_log_file=/var/log/mysql-slow/slow.log
 long_query_time=3   设置慢查询超时时间  单位是：秒
 ```
 创建对应目录：
 ```shell
 [root@xingdian ~]# mkdir /var/log/mysql-slow
 [root@xingdian ~]# chown mysql.mysql mysql-slow
 ```
 重启服务：
 ```shell
 [root@xingdian ~]# systemctl restart mysqld
 ```
 验证：
 ```shell
 [root@xingdian ~]# mysql -uroot -pQianFeng@123
 mysql: [Warning] Using a password on the command line interface can be insecure.
 Welcome to the MySQL monitor.  Commands end with ; or \g.
 Your MySQL connection id is 2
 Server version: 5.7.39-log MySQL Community Server (GPL)
 Copyright (c) 2000, 2022, Oracle and/or its affiliates.
 Oracle is a registered trademark of Oracle Corporation and/or its
 affiliates. Other names may be trademarks of their respective
 owners.
 Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
 mysql> select sleep(6); 
 +----------+
 | sleep(6) |
 +----------+
 |        0 |
 +----------+
 1 row in set (6.00 sec)
 mysql> exit
 Bye
 [root@xingdian ~]# cat /var/log/mysql-slow/slow.log 
 /usr/sbin/mysqld, Version: 5.7.39-log (MySQL Community Server (GPL)). started with:
 Tcp port: 0  Unix socket: /var/lib/mysql/mysql.sock
 Time                 Id Command    Argument
 # Time: 2022-09-25T06:58:05.496205Z
 # User@Host: root[root] @ localhost []  Id:     2
 # Query_time: 6.007094  Lock_time: 0.000000 Rows_sent: 1  Rows_examined: 0
 SET timestamp=1664089085;
 select sleep(6);
 ```