删除 第三章：数据库查询.md
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第三章：数据库查询.md
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第三章：数据库查询.md
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<h1><center>数据库查询</center></h1>
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**作者：行癫（盗版必究）**
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------
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## 一：基本查询
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#### 1.简介
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单表查询
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简单查询
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通过条件查询
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查询排序
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限制查询记录数
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使用集合函数查询
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分组查询
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使用正则表达式查询
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#### 2.案例
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创建案例所需表：company.employee5
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```shell
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雇员编号 id int
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雇员姓名 name varchar(30)
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雇员性别 sex enum
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雇用时期 hire_date date
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职位 post varchar(50)
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职位描述 job_description varchar(100)
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薪水 salary double(15,2)
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办公室 office int
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部门编号 dep_id int
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```
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```shell
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MySQL [(none)]> CREATE TABLE company.employee5(
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id int primary key AUTO_INCREMENT not null,
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name varchar(30) not null,
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sex enum('male','female') default 'male' not null,
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hire_date date not null,
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post varchar(50) not null,
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job_description varchar(100),
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salary double(15,2) not null,
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office int,
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dep_id int
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);
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```
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插入模拟数据：
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```shell
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MySQL [(none)]> insert into company.employee5(name,sex,hire_date,post,job_description,salary,office,dep_id) values
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('jack','male','20180202','instructor','teach',5000,501,100),
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('tom','male','20180203','instructor','teach',5500,501,100),
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('robin','male','20180202','instructor','teach',8000,501,100),
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('alice','female','20180202','instructor','teach',7200,501,100),
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('','male','20180202','hr','hrcc',600,502,101),
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('harry','male','20180202','hr',NULL,6000,502,101),
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('emma','female','20180206','sale','salecc',20000,503,102),
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('christine','female','20180205','sale','salecc',2200,503,102),
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('zhuzhu','male','20180205','sale',NULL,2200,503,102),
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('gougou','male','20180205','sale','',2200,503,102);
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```
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语法格式：
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select 字段名称,字段名称2...... from 表名 [条件]
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##### a.简单查询
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```shell
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MySQL [company]> select * from employee5;
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+----+-----------+--------+------------+------------+-----------------+----------+--------+--------+
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| id | name | sex | hire_date | post | job_description | salary | office | dep_id |
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+----+-----------+--------+------------+------------+-----------------+----------+--------+--------+
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| 1 | jack | male | 2018-02-02 | instructor | teach | 5000.00 | 501 | 100 |
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| 2 | tom | male | 2018-02-03 | instructor | teach | 5500.00 | 501 | 100 |
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| 3 | robin | male | 2018-02-02 | instructor | teach | 8000.00 | 501 | 100 |
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| 4 | alice | female | 2018-02-02 | instructor | teach | 7200.00 | 501 | 100 |
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| 5 | | male | 2018-02-02 | hr | hrcc | 600.00 | 502 | 101 |
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| 6 | harry | male | 2018-02-02 | hr | NULL | 6000.00 | 502 | 101 |
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| 7 | emma | female | 2018-02-06 | sale | salecc | 20000.00 | 503 | 102 |
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| 8 | christine | female | 2018-02-05 | sale | salecc | 2200.00 | 503 | 102 |
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| 9 | zhuzhu | male | 2018-02-05 | sale | NULL | 2200.00 | 503 | 102 |
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| 10 | gougou | male | 2018-02-05 | sale | | 2200.00 | 503 | 102 |
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+----+-----------+--------+------------+------------+-----------------+----------+--------+--------+
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10 rows in set (0.00 sec)
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MySQL [company]> select name, salary, dep_id from employee5 where id <=5;
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+-------+---------+--------+
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| name | salary | dep_id |
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+-------+---------+--------+
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| jack | 5000.00 | 100 |
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| tom | 5500.00 | 100 |
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| robin | 8000.00 | 100 |
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| alice | 7200.00 | 100 |
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| | 600.00 | 101 |
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+-------+---------+--------+
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5 rows in set (0.00 sec)
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```
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##### b.避免重复
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不能部分使用DISTINCT，通常仅用于某一字段
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```shell
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MySQL [company]> SELECT post FROM employee5;
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+------------+
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| post |
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+------------+
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| instructor |
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| instructor |
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| instructor |
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| instructor |
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| hr |
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| hr |
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| sale |
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| sale |
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| sale |
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| sale |
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+------------+
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10 rows in set (0.00 sec)
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MySQL [company]> SELECT distinct post FROM employee5;
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+------------+
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| post |
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+------------+
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| instructor |
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| hr |
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| sale |
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+------------+
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3 rows in set (0.00 sec)
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```
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##### c.四则运算查询
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```shell
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MySQL [company]> SELECT name, salary, salary*14 FROM employee5;
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+-----------+----------+-----------+
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| name | salary | salary*14 |
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+-----------+----------+-----------+
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| jack | 5000.00 | 70000.00 |
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| tom | 5500.00 | 77000.00 |
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| robin | 8000.00 | 112000.00 |
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| alice | 7200.00 | 100800.00 |
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| | 600.00 | 8400.00 |
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| harry | 6000.00 | 84000.00 |
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| emma | 20000.00 | 280000.00 |
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| christine | 2200.00 | 30800.00 |
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| zhuzhu | 2200.00 | 30800.00 |
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| gougou | 2200.00 | 30800.00 |
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+-----------+----------+-----------+
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10 rows in set (0.01 sec)
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MySQL [company]> SELECT name, salary, salary*14 AS Annual_salary FROM employee5;
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+-----------+----------+---------------+
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| name | salary | Annual_salary |
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+-----------+----------+---------------+
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| jack | 5000.00 | 70000.00 |
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| tom | 5500.00 | 77000.00 |
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| robin | 8000.00 | 112000.00 |
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| alice | 7200.00 | 100800.00 |
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| | 600.00 | 8400.00 |
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| harry | 6000.00 | 84000.00 |
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| emma | 20000.00 | 280000.00 |
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| christine | 2200.00 | 30800.00 |
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| zhuzhu | 2200.00 | 30800.00 |
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| gougou | 2200.00 | 30800.00 |
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+-----------+----------+---------------+
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10 rows in set (0.00 sec)
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MySQL [company]> SELECT name, salary, salary*14 Annual_salary FROM employee5;
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+-----------+----------+---------------+
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| name | salary | Annual_salary |
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+-----------+----------+---------------+
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| jack | 5000.00 | 70000.00 |
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| tom | 5500.00 | 77000.00 |
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| robin | 8000.00 | 112000.00 |
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| alice | 7200.00 | 100800.00 |
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| | 600.00 | 8400.00 |
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| harry | 6000.00 | 84000.00 |
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| emma | 20000.00 | 280000.00 |
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| christine | 2200.00 | 30800.00 |
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| zhuzhu | 2200.00 | 30800.00 |
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| gougou | 2200.00 | 30800.00 |
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+-----------+----------+---------------+
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10 rows in set (0.00 sec)
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```
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##### d.定义显示格式
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CONCAT() 函数用于连接字符串
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```shell
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MySQL [company]> SELECT concat(name, 's annual salary: ', salary*14) AS Annual_salary FROM employee5;
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+------------------------------------+
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| Annual_salary |
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+------------------------------------+
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| jacks annual salary: 70000.00 |
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| toms annual salary: 77000.00 |
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| robins annual salary: 112000.00 |
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| alices annual salary: 100800.00 |
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| s annual salary: 8400.00 |
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| harrys annual salary: 84000.00 |
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| emmas annual salary: 280000.00 |
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| christines annual salary: 30800.00 |
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| zhuzhus annual salary: 30800.00 |
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| gougous annual salary: 30800.00 |
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+------------------------------------+
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10 rows in set (0.00 sec)
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```
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##### e.单条件查询
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```shell
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MySQL [company]> SELECT name,post FROM employee5 WHERE post='hr';
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+-------+------+
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| name | post |
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+-------+------+
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| | hr |
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| harry | hr |
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+-------+------+
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2 rows in set (0.00 sec)
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```
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##### f.多条件查询
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```shell
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MySQL [company]> SELECT name,salary FROM employee5 WHERE post='hr' AND salary>10000;
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Empty set (0.00 sec)
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MySQL [company]> select * from employee5 where salary>5000 and salary<10000 or dep_id=102;
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+----+-----------+--------+------------+------------+-----------------+----------+--------+--------+
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| id | name | sex | hire_date | post | job_description | salary | office | dep_id |
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+----+-----------+--------+------------+------------+-----------------+----------+--------+--------+
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| 2 | tom | male | 2018-02-03 | instructor | teach | 5500.00 | 501 | 100 |
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| 3 | robin | male | 2018-02-02 | instructor | teach | 8000.00 | 501 | 100 |
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| 4 | alice | female | 2018-02-02 | instructor | teach | 7200.00 | 501 | 100 |
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| 6 | harry | male | 2018-02-02 | hr | NULL | 6000.00 | 502 | 101 |
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| 7 | emma | female | 2018-02-06 | sale | salecc | 20000.00 | 503 | 102 |
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| 8 | christine | female | 2018-02-05 | sale | salecc | 2200.00 | 503 | 102 |
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| 9 | zhuzhu | male | 2018-02-05 | sale | NULL | 2200.00 | 503 | 102 |
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| 10 | gougou | male | 2018-02-05 | sale | | 2200.00 | 503 | 102 |
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+----+-----------+--------+------------+------------+-----------------+----------+--------+--------+
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8 rows in set (0.00 sec)
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```
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##### g.关键字
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BETWEEN AND
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```shell
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MySQL [company]> SELECT name,salary FROM employee5 WHERE salary BETWEEN 5000 AND 15000;
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+-------+---------+
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| name | salary |
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+-------+---------+
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| jack | 5000.00 |
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| tom | 5500.00 |
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| robin | 8000.00 |
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| alice | 7200.00 |
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| harry | 6000.00 |
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+-------+---------+
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5 rows in set (0.00 sec)
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MySQL [company]> SELECT name,salary FROM employee5 WHERE salary NOT BETWEEN 5000 AND 15000;
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+-----------+----------+
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| name | salary |
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+-----------+----------+
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| | 600.00 |
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| emma | 20000.00 |
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| christine | 2200.00 |
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| zhuzhu | 2200.00 |
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| gougou | 2200.00 |
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+-----------+----------+
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5 rows in set (0.00 sec)
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```
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IS NULL
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```shell
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MySQL [company]> SELECT name,job_description FROM employee5 WHERE job_description IS NULL;
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+--------+-----------------+
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| name | job_description |
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+--------+-----------------+
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| harry | NULL |
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| zhuzhu | NULL |
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+--------+-----------------+
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2 rows in set (0.00 sec)
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MySQL [company]> SELECT name,job_description FROM employee5 WHERE job_description IS NOT NULL;
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+-----------+-----------------+
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| name | job_description |
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+-----------+-----------------+
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| jack | teach |
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| tom | teach |
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| robin | teach |
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| alice | teach |
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| | hrcc |
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| emma | salecc |
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| christine | salecc |
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| gougou | |
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+-----------+-----------------+
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8 rows in set (0.00 sec)
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MySQL [company]> SELECT name,job_description FROM employee5 WHERE job_description='';
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+--------+-----------------+
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| name | job_description |
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+--------+-----------------+
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| gougou | |
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+--------+-----------------+
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1 row in set (0.00 sec)
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```
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注意：（NULL说明）
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等价于没有任何值、是未知数
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NULL与0、空字符串、空格都不同,NULL没有分配存储空间
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对空值做加、减、乘、除等运算操作，结果仍为空
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比较时使用关键字用“is null”和“is not null”
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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
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IN集合查询
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```shell
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MySQL [company]> SELECT name, salary FROM employee5 WHERE salary=4000 OR salary=5000 OR salary=6000 OR salary=9000 ;
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+-------+---------+
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| name | salary |
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+-------+---------+
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| jack | 5000.00 |
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| harry | 6000.00 |
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+-------+---------+
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2 rows in set (0.00 sec)
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MySQL [company]> SELECT name, salary FROM employee5 WHERE salary IN (4000,5000,6000,9000) ;
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+-------+---------+
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| name | salary |
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+-------+---------+
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| jack | 5000.00 |
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| harry | 6000.00 |
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+-------+---------+
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2 rows in set (0.00 sec)
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MySQL [company]> SELECT name, salary FROM employee5 WHERE salary NOT IN (4000,5000,6000,9000) ;
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+-----------+----------+
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| name | salary |
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+-----------+----------+
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| tom | 5500.00 |
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| robin | 8000.00 |
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| alice | 7200.00 |
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| | 600.00 |
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| emma | 20000.00 |
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| christine | 2200.00 |
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| zhuzhu | 2200.00 |
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| gougou | 2200.00 |
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+-----------+----------+
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8 rows in set (0.01 sec)
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```
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##### h.模糊查询
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关键字LIKE
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通配符%：所有字符
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通配符_： 一个字符
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```shell
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MySQL [company]> SELECT * FROM employee5 WHERE name LIKE 'al%';
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+----+-------+--------+------------+------------+-----------------+---------+--------+--------+
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| id | name | sex | hire_date | post | job_description | salary | office | dep_id |
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+----+-------+--------+------------+------------+-----------------+---------+--------+--------+
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| 4 | alice | female | 2018-02-02 | instructor | teach | 7200.00 | 501 | 100 |
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+----+-------+--------+------------+------------+-----------------+---------+--------+--------+
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1 row in set (0.00 sec)
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MySQL [company]> SELECT * FROM employee5 WHERE name LIKE 'al___';
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+----+-------+--------+------------+------------+-----------------+---------+--------+--------+
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| id | name | sex | hire_date | post | job_description | salary | office | dep_id |
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+----+-------+--------+------------+------------+-----------------+---------+--------+--------+
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| 4 | alice | female | 2018-02-02 | instructor | teach | 7200.00 | 501 | 100 |
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+----+-------+--------+------------+------------+-----------------+---------+--------+--------+
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1 row in set (0.00 sec)
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```
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##### i.排序查询
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```shell
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MySQL [company]> select name,salary from employee5 order by salary;
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+-----------+----------+
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| name | salary |
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+-----------+----------+
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| | 600.00 |
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| christine | 2200.00 |
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| zhuzhu | 2200.00 |
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| gougou | 2200.00 |
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| jack | 5000.00 |
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| tom | 5500.00 |
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| harry | 6000.00 |
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| alice | 7200.00 |
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| robin | 8000.00 |
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| emma | 20000.00 |
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+-----------+----------+
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10 rows in set (0.01 sec)
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MySQL [company]> select name,salary from employee5 order by salary desc;
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+-----------+----------+
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| name | salary |
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+-----------+----------+
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| emma | 20000.00 |
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| robin | 8000.00 |
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| alice | 7200.00 |
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| harry | 6000.00 |
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| tom | 5500.00 |
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| jack | 5000.00 |
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| christine | 2200.00 |
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| zhuzhu | 2200.00 |
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| gougou | 2200.00 |
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| | 600.00 |
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+-----------+----------+
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MySQL [company]> select name,salary from employee5 order by salary desc limit 3; //控制显示前3行
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+-------+----------+
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| name | salary |
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+-------+----------+
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| emma | 20000.00 |
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| robin | 8000.00 |
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| alice | 7200.00 |
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+-------+----------+
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3 rows in set (0.00 sec)
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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
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+-------+---------+
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| name | salary |
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+-------+---------+
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| robin | 8000.00 |
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| alice | 7200.00 |
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| harry | 6000.00 |
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+-------+---------+
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3 rows in set (0.00 sec)
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```
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注意：
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||||
|
||||
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)
|
||||
```
|
||||
Loading…
Reference in New Issue