Mysql笔记整理2

2025-01-19 20:16:54 +08:00
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 # Bolg
 > https://duyl328.github.io/
 ## 协议
 本仓库采用双协议授权，即 MIT 协议和 CC-BY-4.0 协议：
 - 所有`.md`文件采用 CC-BY-4.0 协议协议，你需要保留署名权
 - 其他代码文件采用 MIT 协议，你可以自由使用
 具体内容请查看[LICENSE](./LICENSE)文件。
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 ## 练习
 基础：
 	数据库的启停；
 	MySQL的登录（显示密码和不显示密码）；
 	退出MySQL；
 	查看数据库中有哪些库；
 	使用数据库；
 	创建数据库；
 	查看数据库下有多少个表；
 	查看表的结构，不显示表中数据；
 	查看当前使用哪个数据库；
 	删除数据库；
 	如何结束一个sql语句（不使用’；‘）
 	查看MySQL版本号；
 	查看创建表时使用的语句；
 	sql数据导入；
 	查看表中全部数据；
 查询：
 	简单查询；
 	查询字段的简单的数学运算；
 	给查询后的列重命名；
 	给查询后的列使用中文名字命名；
 	AS关键字省略写法；
 条件查询：
 	语法；
 		查询工资等于5000的员工姓名；
 		查询SMITH工资；
 		查询员工工资大于3000的员工；
 		查询员工工资高于1100并低于3000的员工姓名及工资；（两种查询方式）
 	is null：
 		找出哪些员工的福利（COMM）为NULL；
 	in ， not in：
 		使用in找出工作是(MANAGER和SALESMAN）的员工；
 \-------------------------------------------------------------------------------------------------
 	模糊查询：
 		查询名字中含有字母o的；
 		查询名字中第二个字母是A的；
 		查询名字中含有 ‘ _ ’的；
 		查询名字最后一个字母是T的；
 	排序：
 		数据排序：
 			找出岗位是salesman的员工，并按照工资降序排列；
 			按照工资升序排，当工资相同时按照名字降序排；
 函数：
 	分组函数：
 	找出最高、最低、平均、工资总和
 	分组函数对NULL的处理；
 	单行处理函数对NULL的处理；
 	单行处理函数如何处理NULL；
 	count * 和count（字段名）的区别；
 	找出工资高于平均工资的员工；
 	找出每个岗位的最高薪资；
 	select、from、where、group by、having、order by的执行顺序；
 	找出不同部门每个工作岗位的最高薪资；
 	找出每个部门的最高薪资，要求显示大于2500的数据；（两种方式【where，having】）
 	查询结果的去重；
 连接查询:
 	内连接：
 		什么是笛卡尔积现象；
 		笛卡尔积现象能避免吗；
 		查询每个员工部门名称，要求显示员工名和部门名；
 		找出每个员工的工资等级，要求显示员工名、工资、工资等级；
 		找出每个员工对应的上级领导，要求显示员工名和领导名；
 	外连接：
 		找出每个员工的上级领导，没有也要显示（14条记录）
 		找出哪个部门没有员工；
 		为什么要为每个表取别名    
 	三表联查：
 		找出每一个员工的部门名称以及工资等级
 		找出每一个员工的部门名称以及工资等级，以及上级领导
 ## 1 - 5
 #### 1、取得每个部门最高薪水的人员名称
 第一步：取得每个部门最高薪水(按照部门编号分组，找出每一组最大值)
 ```sql
 mysql> select deptno,max(sal) as maxsal from emp group by deptno;
 +--------+---------+
 | deptno | maxsal  |
 +--------+---------+
 |     10 | 5000.00 |
 |     20 | 3000.00 |
 |     30 | 2850.00 |
 +--------+---------+
 ```
 第二步：将以上的查询结果当做一张临时表t，
 t 和 emp 表连接，条件：t.deptno = e.deptno and t.maxsal = e.sal
 ```sql
 select
    e.ename, t.*
 from
    emp e
 join
    (select deptno,max(sal) as maxsal from emp group by deptno) t
 on
    t.deptno = e.deptno and t.maxsal = e.sal;
 +-------+--------+---------+
 | ename | deptno | maxsal  |
 +-------+--------+---------+
 | BLAKE |     30 | 2850.00 |
 | SCOTT |     20 | 3000.00 |
 | KING  |     10 | 5000.00 |
 | FORD  |     20 | 3000.00 |
 +-------+--------+---------+
 ```
 #### 2、哪些人的薪水在部门的平均薪水之上
 第一步：找出每个部门的平均薪水
 ```sql
 select deptno,avg(sal) as avgsal from emp group by deptno;
 +--------+-------------+
 | deptno | avgsal      |
 +--------+-------------+
 |     10 | 2916.666667 |
 |     20 | 2175.000000 |
 |     30 | 1566.666667 |
 +--------+-------------+
 ```
 第二步：将以上查询结果当做t表，t和emp表连接
 条件：部门编号相同，并且 emp 的 sal 大于 t 表的 avgsal
 ```sql
 select
    t.*, e.ename, e.sal
 from
    emp e
 join
    (select deptno,avg(sal) as avgsal from emp group by deptno) t
 on
    e.deptno = t.deptno and e.sal > t.avgsal;
 +--------+-------------+-------+---------+
 | deptno | avgsal      | ename | sal     |
 +--------+-------------+-------+---------+
 |     30 | 1566.666667 | ALLEN | 1600.00 |
 |     20 | 2175.000000 | JONES | 2975.00 |
 |     30 | 1566.666667 | BLAKE | 2850.00 |
 |     20 | 2175.000000 | SCOTT | 3000.00 |
 |     10 | 2916.666667 | KING  | 5000.00 |
 |     20 | 2175.000000 | FORD  | 3000.00 |
 +--------+-------------+-------+---------+
 ```
 #### 3、取得部门中（所有人的）平均的薪水等级
 平均的薪水等级：先计算每一个薪水的等级，然后找出薪水等级的平均值。
 平均薪水的等级：先计算平均薪水，然后找出每个平均薪水的等级值。
 第一步：找出每个人的薪水等级
 emp e 和 salgrade s 表连接。
 连接条件：e.sal between s.losal and s.hisal
 ```sql
 select 
 	e.ename,e.sal,e.deptno,s.grade
 from
 	emp e
 join
 	salgrade s
 on
 	e.sal between s.losal and s.hisal;
 +--------+---------+--------+-------+
 | ename  | sal     | deptno | grade |
 +--------+---------+--------+-------+
 | CLARK  | 2450.00 |     10 |     4 |
 | KING   | 5000.00 |     10 |     5 |
 | MILLER | 1300.00 |     10 |     2 |
 | SMITH  |  800.00 |     20 |     1 |
 | ADAMS  | 1100.00 |     20 |     1 |
 | SCOTT  | 3000.00 |     20 |     4 |
 | FORD   | 3000.00 |     20 |     4 |
 | JONES  | 2975.00 |     20 |     4 |
 | MARTIN | 1250.00 |     30 |     2 |
 | TURNER | 1500.00 |     30 |     3 |
 | BLAKE  | 2850.00 |     30 |     4 |
 | ALLEN  | 1600.00 |     30 |     3 |
 | JAMES  |  950.00 |     30 |     1 |
 | WARD   | 1250.00 |     30 |     2 |
 +--------+---------+--------+-------+
 ```
 第二步：基于以上的结果继续按照deptno分组，求grade的平均值。
 ```sql
 select 
 	e.deptno,avg(s.grade)
 from
 	emp e
 join
 	salgrade s
 on
 	e.sal between s.losal and s.hisal
 group by
 	e.deptno;
 +--------+--------------+
 | deptno | avg(s.grade) |
 +--------+--------------+
 |     10 |       3.6667 |
 |     20 |       2.8000 |
 |     30 |       2.5000 |
 +--------+--------------+
 ```
 #### 4、不准用组函数（Max ），取得最高薪水
 第一种：sal 降序，limit 1
 ```sql
 select ename,sal from emp order by sal desc limit 1;
 +-------+---------+
 | ename | sal     |
 +-------+---------+
 | KING  | 5000.00 |
 +-------+---------+
 ```
 第二种方案：
 ```sql
 select max(sal) from emp;
 ```
 第三种方案：表的自连接
 ```sql
 select sal from emp where sal not in(select distinct a.sal from emp a join emp b on a.sal < b.sal);
 +---------+
 | sal     |
 +---------+
 | 5000.00 |
 +---------+
 ```
 ```sql
 select
 distinct a.sal
 from
 emp a
 join
 emp b
 on
 a.sal < b.sal
 ```
 a表
 ```sql
 +---------+
 | sal     |
 +---------+
 |  800.00 |
 | 1600.00 |
 | 1250.00 |
 | 2975.00 |
 | 1250.00 |
 | 2850.00 |
 | 2450.00 |
 | 3000.00 |
 | 5000.00 |
 | 1500.00 |
 | 1100.00 |
 |  950.00 |
 | 3000.00 |
 | 1300.00 |
 +---------+
 ```
 b表
 ```sql
 +---------+
 | sal     |
 +---------+
 |  800.00 |
 | 1600.00 |
 | 1250.00 |
 | 2975.00 |
 | 1250.00 |
 | 2850.00 |
 | 2450.00 |
 | 3000.00 |
 | 5000.00 |
 | 1500.00 |
 | 1100.00 |
 |  950.00 |
 | 3000.00 |
 | 1300.00 |
 +---------+
 ```
 #### 5、取得平均薪水最高的部门的部门编号
 第一种方案：降序取第一个。
 	第一步：找出每个部门的平均薪水
 ```sql
 select deptno,avg(sal) as avgsal from emp group by deptno;
 +--------+-------------+
 | deptno | avgsal      |
 +--------+-------------+
 |     10 | 2916.666667 |
 |     20 | 2175.000000 |
 |     30 | 1566.666667 |
 +--------+-------------+
 ```
 	第二步：降序选第一个。
 ```sql
 select deptno,avg(sal) as avgsal from emp group by deptno order by avgsal desc limit 1;
 +--------+-------------+
 | deptno | avgsal      |
 +--------+-------------+
 |     10 | 2916.666667 |
 +--------+-------------+
 ```
 第二种方案：max
 	第一步：找出每个部门的平均薪水
 ```sql
 select deptno,avg(sal) as avgsal from emp group by deptno;
 +--------+-------------+
 | deptno | avgsal      |
 +--------+-------------+
 |     10 | 2916.666667 |
 |     20 | 2175.000000 |
 |     30 | 1566.666667 |
 +--------+-------------+
 ```
 	第二步：找出以上结果中avgsal最大的值。
 ```sql
 select max(t.avgsal) from (select avg(sal) as avgsal from emp group by deptno) t;
 +---------------+
 | max(t.avgsal) |
 +---------------+
 |   2916.666667 |
 +---------------+
 ```
 	第三步：
 ```sql
 select 
 	deptno,avg(sal) as avgsal 
 from 
 	emp 
 group by 
 	deptno
 having
 	avgsal = (select max(t.avgsal) from (select avg(sal) as avgsal from emp group by deptno) t);
 +--------+-------------+
 | deptno | avgsal      |
 +--------+-------------+
 |     10 | 2916.666667 |
 +--------+-------------+
 ```
 ## 5 - 10
 #### 6、取得平均薪水最高的部门的部门名称
 ```sql
 select
    d.dname,avg(e.sal) as avgsal
 from
    emp e
 join
    dept d
 on
    e.deptno = d.deptno
 group by
    d.dname
 order by
    avgsal desc
 limit
    1;
 +------------+-------------+
 | dname      | avgsal      |
 +------------+-------------+
 | ACCOUNTING | 2916.666667 |
 +------------+-------------+
 ```
 #### 7、求平均薪水的等级最低的部门的部门名称
 平均薪水是 800
 平均薪水是 900
 那么他俩都是 1 级别。
 第一步：找出每个部门的平均薪水
 ```sql
 select deptno,avg(sal) as avgsal from emp group by deptno;
 +--------+-------------+
 | deptno | avgsal      |
 +--------+-------------+
 |     10 | 2916.666667 |
 |     20 | 2175.000000 |
 |     30 | 1566.666667 |
 +--------+-------------+
 ```
 第二步：找出每个部门的平均薪水的等级
 以上 t 表和 salgrade 表连接，条件：t.avgsal between s.losal and s.hisal
 ```sql
 select
    t.*,s.grade
 from
    (select d.dname,avg(sal) as avgsal from emp e join dept d on e.deptno = d.deptno group by d.dname) t
 join
    salgrade s
 on
    t.avgsal between s.losal and s.hisal;
 +------------+-------------+-------+
 | dname      | avgsal      | grade |
 +------------+-------------+-------+
 | SALES      | 1566.666667 |     3 |
 | ACCOUNTING | 2916.666667 |     4 |
 | RESEARCH   | 2175.000000 |     4 |
 +------------+-------------+-------+
 ```
 ```sql
 select
    t.*,s.grade
 from
    (select d.dname,avg(sal) as avgsal from emp e join dept d on e.deptno = d.deptno group by d.dname) t
 join
    salgrade s
 on
    t.avgsal between s.losal and s.hisal
 where
    s.grade = (select grade from salgrade where (select avg(sal) as avgsal from emp group by deptno order by avgsal asc limit 1) between losal and hisal);
 +-------+-------------+-------+
 | dname | avgsal      | grade |
 +-------+-------------+-------+
 | SALES | 1566.666667 |     3 |
 +-------+-------------+-------+
 ```
 抛开之前的，最低等级你怎么着？
 平均薪水最低的对应的等级一定是最低的.
 ```sql
 select avg(sal) as avgsal from emp group by deptno order by avgsal asc limit 1;
 +-------------+
 | avgsal      |
 +-------------+
 | 1566.666667 |
 +-------------+
 ```
 ```sql
 select grade from salgrade where (select avg(sal) as avgsal from emp group by deptno order by avgsal asc limit 1) between losal and hisal;
 +-------+
 | grade |
 +-------+
 |     3 |
 +-------+
 ```
 #### 8、取得比普通员工(员工代码没有在 mgr 字段上出现的) 的最高薪水还要高的领导人姓名
 > 比“普通员工的最高薪水”还要高的一定是领导！
 > 	没毛病！！！！
 ```sql
 mysql> select distinct mgr from emp where mgr is not null;
 +------+
 | mgr  |
 +------+
 | 7902 |
 | 7698 |
 | 7839 |
 | 7566 |
 | 7788 |
 | 7782 |
 +------+
 ```
 员工编号没有在以上范围内的都是普通员工。
 第一步：找出普通员工的最高薪水！
 not in 在使用的时候，后面小括号中记得排除NULL。
 ```sql
 select max(sal) from emp where empno not in(select distinct mgr from emp where mgr is not null);
 +----------+
 | max(sal) |
 +----------+
 |  1600.00 |
 +----------+
 ```
 第二步：找出高于1600的
 ```sql
 select ename,sal from emp where sal > (select max(sal) from emp where empno not in(select distinct mgr from emp where mgr is not null));
 +-------+---------+
 | ename | sal     |
 +-------+---------+
 | JONES | 2975.00 |
 | BLAKE | 2850.00 |
 | CLARK | 2450.00 |
 | SCOTT | 3000.00 |
 | KING  | 5000.00 |
 | FORD  | 3000.00 |
 +-------+---------+
 ```
 #### 9、取得薪水最高的前五名员工
 ```sql
 select ename,sal from emp order by sal desc limit 5;
 +-------+---------+
 | ename | sal     |
 +-------+---------+
 | KING  | 5000.00 |
 | SCOTT | 3000.00 |
 | FORD  | 3000.00 |
 | JONES | 2975.00 |
 | BLAKE | 2850.00 |
 +-------+---------+
 ```
 #### 10、取得薪水最高的第六到第十名员工
 ```sql
 select ename,sal from emp order by sal desc limit 5;
 +-------+---------+
 | ename | sal     |
 +-------+---------+
 | KING  | 5000.00 |
 | SCOTT | 3000.00 |
 | FORD  | 3000.00 |
 | JONES | 2975.00 |
 | BLAKE | 2850.00 |
 +-------+---------+
 select ename,sal from emp order by sal desc limit 5, 5;
 +--------+---------+
 | ename  | sal     |
 +--------+---------+
 | CLARK  | 2450.00 |
 | ALLEN  | 1600.00 |
 | TURNER | 1500.00 |
 | MILLER | 1300.00 |
 | MARTIN | 1250.00 |
 +--------+---------+
 ```
 ## 11 - 15
 #### 11、取得最后入职的 5 名员工
 日期也可以降序，升序。
 ```sql
 select ename,hiredate from emp order by hiredate desc limit 5;
 +--------+------------+
 | ename  | hiredate   |
 +--------+------------+
 | ADAMS  | 1987-05-23 |
 | SCOTT  | 1987-04-19 |
 | MILLER | 1982-01-23 |
 | FORD   | 1981-12-03 |
 | JAMES  | 1981-12-03 |
 +--------+------------+
 ```
 #### 12、取得每个薪水等级有多少员工
 分组 count
 第一步：找出每个员工的薪水等级
 ```sql
 select
 e.ename,e.sal,s.grade
 from
 emp e
 join
 salgrade s
 on
 e.sal between s.losal and s.hisal;
 +--------+---------+-------+
 | ename  | sal     | grade |
 +--------+---------+-------+
 | SMITH  |  800.00 |     1 |
 | ALLEN  | 1600.00 |     3 |
 | WARD   | 1250.00 |     2 |
 | JONES  | 2975.00 |     4 |
 | MARTIN | 1250.00 |     2 |
 | BLAKE  | 2850.00 |     4 |
 | CLARK  | 2450.00 |     4 |
 | SCOTT  | 3000.00 |     4 |
 | KING   | 5000.00 |     5 |
 | TURNER | 1500.00 |     3 |
 | ADAMS  | 1100.00 |     1 |
 | JAMES  |  950.00 |     1 |
 | FORD   | 3000.00 |     4 |
 | MILLER | 1300.00 |     2 |
 +--------+---------+-------+
 ```
 第二步：继续按照grade分组统计数量
 ```sql
 select
 s.grade ,count(*)
 from
 emp e
 join
 salgrade s
 on
 e.sal between s.losal and s.hisal
 group by
 s.grade;
 +-------+----------+
 | grade | count(*) |
 +-------+----------+
 |     1 |        3 |
 |     2 |        3 |
 |     3 |        2 |
 |     4 |        5 |
 |     5 |        1 |
 +-------+----------+
 ```
 #### 13、面试题：
 有 3 个表 S(学生表)，C（课程表），SC（学生选课表）
 S（SNO，SNAME）代表（学号，姓名）
 C（CNO，CNAME，CTEACHER）代表（课号，课名，教师）
 SC（SNO，CNO，SCGRADE）代表（学号，课号，成绩）
 问题：
 1，找出没选过“黎明”老师的所有学生姓名。
 2，列出 2 门以上（含2 门）不及格学生姓名及平均成绩。
 3，即学过 1 号课程又学过 2 号课所有学生的姓名。
 #### 14、列出所有员工及领导的姓名
 ```sql
 select
 a.ename '员工', b.ename '领导'
 from
 emp a
 left join
 emp b
 on
 a.mgr = b.empno;
 +--------+-------+
 | 员工   | 领导    |
 +--------+-------+
 | SMITH  | FORD  |
 | ALLEN  | BLAKE |
 | WARD   | BLAKE |
 | JONES  | KING  |
 | MARTIN | BLAKE |
 | BLAKE  | KING  |
 | CLARK  | KING  |
 | SCOTT  | JONES |
 | KING   | NULL  |
 | TURNER | BLAKE |
 | ADAMS  | SCOTT |
 | JAMES  | BLAKE |
 | FORD   | JONES |
 | MILLER | CLARK |
 +--------+-------+
 ```
 #### 15、列出受雇日期早于其直接上级的所有员工的编号,姓名,部门名称
 ```sql
 emp a 员工表
 emp b 领导表
 a.mgr = b.empno and a.hiredate < b.hiredate
 select
 a.ename '员工', a.hiredate, b.ename '领导', b.hiredate, d.dname
 from
 emp a
 join
 emp b
 on
 a.mgr = b.empno
 join
 dept d
 on
 a.deptno = d.deptno
 where
 a.hiredate < b.hiredate;
 +-------+------------+-------+------------+------------+
 | 员工     | hiredate   | 领导    | hiredate   | dname      |
 +-------+------------+-------+------------+------------+
 | CLARK | 1981-06-09 | KING  | 1981-11-17 | ACCOUNTING |
 | SMITH | 1980-12-17 | FORD  | 1981-12-03 | RESEARCH   |
 | JONES | 1981-04-02 | KING  | 1981-11-17 | RESEARCH   |
 | ALLEN | 1981-02-20 | BLAKE | 1981-05-01 | SALES      |
 | WARD  | 1981-02-22 | BLAKE | 1981-05-01 | SALES      |
 | BLAKE | 1981-05-01 | KING  | 1981-11-17 | SALES      |
 +-------+------------+-------+------------+------------+
 ```
 ## 16 - 20
 #### 16、 列出部门名称和这些部门的员工信息, 同时列出那些没有员工的部门
 ```sql
 select
 e.*,d.dname
 from
 emp e
 right join
 dept d
 on
 e.deptno = d.deptno;
 +-------+--------+-----------+------+------------+---------+---------+--------+------------+
 | EMPNO | ENAME  | JOB       | MGR  | HIREDATE   | SAL     | COMM    | DEPTNO | dname      |
 +-------+--------+-----------+------+------------+---------+---------+--------+------------+
 |  7782 | CLARK  | MANAGER   | 7839 | 1981-06-09 | 2450.00 |    NULL |     10 | ACCOUNTING |
 |  7839 | KING   | PRESIDENT | NULL | 1981-11-17 | 5000.00 |    NULL |     10 | ACCOUNTING |
 |  7934 | MILLER | CLERK     | 7782 | 1982-01-23 | 1300.00 |    NULL |     10 | ACCOUNTING |
 |  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800.00 |    NULL |     20 | RESEARCH   |
 |  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 | 2975.00 |    NULL |     20 | RESEARCH   |
 |  7788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 | 3000.00 |    NULL |     20 | RESEARCH   |
 |  7876 | ADAMS  | CLERK     | 7788 | 1987-05-23 | 1100.00 |    NULL |     20 | RESEARCH   |
 |  7902 | FORD   | ANALYST   | 7566 | 1981-12-03 | 3000.00 |    NULL |     20 | RESEARCH   |
 |  7499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 | 1600.00 |  300.00 |     30 | SALES      |
 |  7521 | WARD   | SALESMAN  | 7698 | 1981-02-22 | 1250.00 |  500.00 |     30 | SALES      |
 |  7654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 | 1250.00 | 1400.00 |     30 | SALES      |
 |  7698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 | 2850.00 |    NULL |     30 | SALES      |
 |  7844 | TURNER | SALESMAN  | 7698 | 1981-09-08 | 1500.00 |    0.00 |     30 | SALES      |
 |  7900 | JAMES  | CLERK     | 7698 | 1981-12-03 |  950.00 |    NULL |     30 | SALES      |
 |  NULL | NULL   | NULL      | NULL | NULL       |    NULL |    NULL |   NULL | OPERATIONS |
 +-------+--------+-----------+------+------------+---------+---------+--------+------------+
 ```
 #### 17、列出至少有 5 个员工的所有部门
 按照部门编号分组，计数，筛选出 >= 5
 ```sql
 select
    deptno
 from
    emp
 group by
    deptno
 having
    count(*) >= 5;
 +--------+
 | deptno |
 +--------+
 |     20 |
 |     30 |
 +--------+
 ```
 #### 18、列出薪金比"SMITH" 多的所有员工信息
 ```sql
 select ename,sal from emp where sal > (select sal from emp where ename = 'SMITH');
 +--------+---------+
 | ename  | sal     |
 +--------+---------+
 | ALLEN  | 1600.00 |
 | WARD   | 1250.00 |
 | JONES  | 2975.00 |
 | MARTIN | 1250.00 |
 | BLAKE  | 2850.00 |
 | CLARK  | 2450.00 |
 | SCOTT  | 3000.00 |
 | KING   | 5000.00 |
 | TURNER | 1500.00 |
 | ADAMS  | 1100.00 |
 | JAMES  |  950.00 |
 | FORD   | 3000.00 |
 | MILLER | 1300.00 |
 +--------+---------+
 ```
 #### 19、 列出所有"CLERK"( 办事员) 的姓名及其部门名称, 部门的人数
 ```sql
 select ename,job from emp where job = 'CLERK';
 +--------+-------+
 | ename  | job   |
 +--------+-------+
 | SMITH  | CLERK |
 | ADAMS  | CLERK |
 | JAMES  | CLERK |
 | MILLER | CLERK |
 +--------+-------+
 select
    e.ename,e.job,d.dname
 from
    emp e
 join
    dept d
 on
    e.deptno = d.deptno
 where
    e.job = 'CLERK';
 +--------+-------+------------+
 | ename  | job   | dname      |
 +--------+-------+------------+
 | MILLER | CLERK | ACCOUNTING |
 | SMITH  | CLERK | RESEARCH   |
 | ADAMS  | CLERK | RESEARCH   |
 | JAMES  | CLERK | SALES      |
 +--------+-------+------------+
 select
    e.ename,e.job,d.dname,d.deptno
 from
    emp e
 join
    dept d
 on
    e.deptno = d.deptno
 where
    e.job = 'CLERK';
 +--------+-------+------------+--------+
 | ename  | job   | dname      | deptno |
 +--------+-------+------------+--------+
 | MILLER | CLERK | ACCOUNTING |     10 |
 | SMITH  | CLERK | RESEARCH   |     20 |
 | ADAMS  | CLERK | RESEARCH   |     20 |
 | JAMES  | CLERK | SALES      |     30 |
 +--------+-------+------------+--------+
 //每个部门的人数？
 select deptno, count(*) as deptcount from emp group by deptno;
 +--------+-----------+
 | deptno | deptcount |
 +--------+-----------+
 |     10 |         3 |
 |     20 |         5 |
 |     30 |         6 |
 +--------+-----------+
 select
    t1.*,t2.deptcount
 from
    (select
        e.ename,e.job,d.dname,d.deptno
    from
        emp e
    join
        dept d
    on
        e.deptno = d.deptno
    where
        e.job = 'CLERK') t1
 join
     (select deptno, count(*) as deptcount from emp group by deptno) t2
 on
     t1.deptno = t2.deptno;
 +--------+-------+------------+--------+-----------+
 | ename  | job   | dname      | deptno | deptcount |
 +--------+-------+------------+--------+-----------+
 | MILLER | CLERK | ACCOUNTING |     10 |         3 |
 | SMITH  | CLERK | RESEARCH   |     20 |         5 |
 | ADAMS  | CLERK | RESEARCH   |     20 |         5 |
 | JAMES  | CLERK | SALES      |     30 |         6 |
 +--------+-------+------------+--------+-----------+
 ```
 #### 20、列出最低薪金大于 1500 的各种工作及从事此工作的全部雇员人数
 按照工作岗位分组求最小值。
 ```sql
 select job,count(*) from emp group by job having min(sal) > 1500;
 +-----------+----------+
 | job       | count(*) |
 +-----------+----------+
 | ANALYST   |        2 |
 | MANAGER   |        3 |
 | PRESIDENT |        1 |
 +-----------+----------+
 ```
 ## 21 - 25
 #### 21、列出在部门"SALES"< 销售部> 工作的员工的姓名, 假定不知道销售部的部门编号.
 ```sql
 select ename from emp where deptno = (select deptno from dept where dname = 'SALES');
 +--------+
 | ename  |
 +--------+
 | ALLEN  |
 | WARD   |
 | MARTIN |
 | BLAKE  |
 | TURNER |
 | JAMES  |
 +--------+
 ```
 #### 22、列出薪金高于公司平均薪金的所有员工, 所在部门, 上级领导, 雇员的工资等级.
 ```sql
 select
    e.ename '员工',d.dname,l.ename '领导',s.grade
 from
    emp e
 join
    dept d
 on
    e.deptno = d.deptno
 left join
    emp l
 on
    e.mgr = l.empno
 join
    salgrade s
 on
    e.sal between s.losal and s.hisal
 where
    e.sal > (select avg(sal) from emp);
 +-------+------------+-------+-------+
 | 员工     | dname      | 领导    | grade |
 +-------+------------+-------+-------+
 | JONES | RESEARCH   | KING  |     4 |
 | BLAKE | SALES      | KING  |     4 |
 | CLARK | ACCOUNTING | KING  |     4 |
 | SCOTT | RESEARCH   | JONES |     4 |
 | KING  | ACCOUNTING | NULL  |     5 |
 | FORD  | RESEARCH   | JONES |     4 |
 +-------+------------+-------+-------+
 ```
 #### 23、 列出与"SCOTT" 从事相同工作的所有员工及部门名称
 ```sql
 select job from emp where ename = 'SCOTT';
 +---------+
 | job     |
 +---------+
 | ANALYST |
 +---------+
 select
    e.ename,e.job,d.dname
 from
    emp e
 join
    dept d
 on
    e.deptno = d.deptno
 where
    e.job = (select job from emp where ename = 'SCOTT')
 and
    e.ename <> 'SCOTT';
 +-------+---------+----------+
 | ename | job     | dname    |
 +-------+---------+----------+
 | FORD  | ANALYST | RESEARCH |
 +-------+---------+----------+
 ```
 #### 24、列出薪金等于部门 30 中员工的薪金的其他员工的姓名和薪金.
 ```sql
 select distinct sal from emp where deptno = 30;
 +---------+
 | sal     |
 +---------+
 | 1600.00 |
 | 1250.00 |
 | 2850.00 |
 | 1500.00 |
 |  950.00 |
 +---------+
 select
    ename,sal
 from
    emp
 where
    sal in(select distinct sal from emp where deptno = 30)
 and
    deptno <> 30;
 Empty set (0.00 sec)
 ```
 #### 25、列出薪金高于在部门 30 工作的所有员工的薪金的员工姓名和薪金. 部门名称
 ```sql
 select max(sal) from emp where deptno = 30;
 +----------+
 | max(sal) |
 +----------+
 |  2850.00 |
 +----------+
 select
    e.ename,e.sal,d.dname
 from
    emp e
 join
    dept d
 on
    e.deptno = d.deptno
 where
    e.sal > (select max(sal) from emp where deptno = 30);
 +-------+---------+------------+
 | ename | sal     | dname      |
 +-------+---------+------------+
 | KING  | 5000.00 | ACCOUNTING |
 | JONES | 2975.00 | RESEARCH   |
 | SCOTT | 3000.00 | RESEARCH   |
 | FORD  | 3000.00 | RESEARCH   |
 +-------+---------+------------+
 ```
 #### 26 - 30
 #### 26、列出在每个部门工作的员工数量, 平均工资和平均服务期限
 没有员工的部门，部门人数是 0
 ```sql
 select
    d.deptno, count(e.ename) ecount,ifnull(avg(e.sal),0) as avgsal, if null(avg(timestampdiff(YEAR, hiredate, now())), 0) as avgservicetime
 from
    emp e
 right join
    dept d
 on
    e.deptno = d.deptno
 group by
    d.deptno;
 +--------+--------+-------------+----------------+
 | deptno | ecount | avgsal      | avgservicetime |
 +--------+--------+-------------+----------------+
 |     10 |      3 | 2916.666667 |        38.0000 |
 |     20 |      5 | 2175.000000 |        35.8000 |
 |     30 |      6 | 1566.666667 |        38.3333 |
 |     40 |      0 |    0.000000 |         0.0000 |
 +--------+--------+-------------+----------------+
 ```
 在 mysql 当中怎么计算两个日期的“年差”，差了多少年？
 TimeStampDiff (间隔类型, 前一个日期, 后一个日期)
 ```sql
 timestampdiff(YEAR, hiredate, now())
 间隔类型：
 	SECOND   秒，
 	MINUTE   分钟，
 	HOUR   小时，
 	DAY   天，
 	WEEK   星期
 	MONTH   月，
 	QUARTER   季度，
 	YEAR   年
 ```
 #### 27、 列出所有员工的姓名、部门名称和工资。
 ```sql
 select
    e.ename,d.dname,e.sal
 from
    emp e
 join
    dept d
 on
    e.deptno = d.deptno;
 +--------+------------+---------+
 | ename  | dname      | sal     |
 +--------+------------+---------+
 | CLARK  | ACCOUNTING | 2450.00 |
 | KING   | ACCOUNTING | 5000.00 |
 | MILLER | ACCOUNTING | 1300.00 |
 | SMITH  | RESEARCH   |  800.00 |
 | JONES  | RESEARCH   | 2975.00 |
 | SCOTT  | RESEARCH   | 3000.00 |
 | ADAMS  | RESEARCH   | 1100.00 |
 | FORD   | RESEARCH   | 3000.00 |
 | ALLEN  | SALES      | 1600.00 |
 | WARD   | SALES      | 1250.00 |
 | MARTIN | SALES      | 1250.00 |
 | BLAKE  | SALES      | 2850.00 |
 | TURNER | SALES      | 1500.00 |
 | JAMES  | SALES      |  950.00 |
 +--------+------------+---------+
 ```
 #### 28、列出所有部门的详细信息和人数
 ```sql
 select
    d.deptno,d.dname,d.loc,count(e.ename)
 from
    emp e
 right join
    dept d
 on
    e.deptno = d.deptno
 group by
    d.deptno,d.dname,d.loc;
 +--------+------------+----------+----------------+
 | deptno | dname      | loc      | count(e.ename) |
 +--------+------------+----------+----------------+
 |     10 | ACCOUNTING | NEW YORK |              3 |
 |     20 | RESEARCH   | DALLAS   |              5 |
 |     30 | SALES      | CHICAGO  |              6 |
 |     40 | OPERATIONS | BOSTON   |              0 |
 +--------+------------+----------+----------------+
 ```
 #### 29、列出各种工作的最低工资及从事此工作的雇员姓名
 ```sql
 select
    job,min(sal) as minsal
 from
    emp
 group by
    job;
 +-----------+----------+
 | job       | minsal		|
 +-----------+----------+
 | ANALYST   |  3000.00 |
 | CLERK     |   800.00 |
 | MANAGER   |  2450.00 |
 | PRESIDENT |  5000.00 |
 | SALESMAN  |  1250.00 |
 +-----------+----------+
 emp e 和以上 t 连接
 select
    e.ename,t.*
 from
    emp e
 join
    (select
        job,min(sal) as minsal
    from
        emp
    group by
        job) t
 on
    e.job = t.job and e.sal = t.minsal;
 +--------+-----------+---------+
 | ename  | job       | minsal  |
 +--------+-----------+---------+
 | SMITH  | CLERK     |  800.00 |
 | WARD   | SALESMAN  | 1250.00 |
 | MARTIN | SALESMAN  | 1250.00 |
 | CLARK  | MANAGER   | 2450.00 |
 | SCOTT  | ANALYST   | 3000.00 |
 | KING   | PRESIDENT | 5000.00 |
 | FORD   | ANALYST   | 3000.00 |
 +--------+-----------+---------+
 ```
 #### 30、列出各个部门的 MANAGER( 领导) 的最低薪金
 ```sql
 select
    deptno, min(sal)
 from
    emp
 where
    job = 'MANAGER'
 group by
    deptno;
 +--------+----------+
 | deptno | min(sal) |
 +--------+----------+
 |     10 |  2450.00 |
 |     20 |  2975.00 |
 |     30 |  2850.00 |
 +--------+----------+
 ```
 ## 30 - 34
 #### 31、列出所有员工的 年工资, 按 年薪从低到高排序
 ```sql
 select
    ename,(sal + ifnull(comm,0)) * 12 as yearsal
 from
    emp
 order by
    yearsal asc;
 +--------+----------+
 | ename  | yearsal  |
 +--------+----------+
 | SMITH  |  9600.00 |
 | JAMES  | 11400.00 |
 | ADAMS  | 13200.00 |
 | MILLER | 15600.00 |
 | TURNER | 18000.00 |
 | WARD   | 21000.00 |
 | ALLEN  | 22800.00 |
 | CLARK  | 29400.00 |
 | MARTIN | 31800.00 |
 | BLAKE  | 34200.00 |
 | JONES  | 35700.00 |
 | FORD   | 36000.00 |
 | SCOTT  | 36000.00 |
 | KING   | 60000.00 |
 +--------+----------+
 ```
 #### 32、求出员工领导的薪水超过3000的员工名称与领导
 ```sql
 select
    a.ename '员工',b.ename '领导'
 from
    emp a
 join
    emp b
 on
    a.mgr = b.empno
 where
    b.sal > 3000;
 +-------+------+
 | 员工  | 领导   |
 +-------+------+
 | JONES | KING |
 | BLAKE | KING |
 | CLARK | KING |
 +-------+------+
 ```
 #### 33、求出部门名称中, 带'S'字符的部门员工的工资合计、部门人数
 ```sql
 select
    d.deptno,d.dname,d.loc,count(e.ename),ifnull(sum(e.sal),0) as sumsal
 from
    emp e
 right join
    dept d
 on
    e.deptno = d.deptno
 where
    d.dname like '%S%'
 group by
    d.deptno,d.dname,d.loc;
 +--------+------------+---------+----------------+----------+
 | deptno | dname      | loc     | count(e.ename) | sumsal   |
 +--------+------------+---------+----------------+----------+
 |     20 | RESEARCH   | DALLAS  |              5 | 10875.00 |
 |     30 | SALES      | CHICAGO |              6 |  9400.00 |
 |     40 | OPERATIONS | BOSTON  |              0 |     0.00 |
 +--------+------------+---------+----------------+----------+
 ```
 #### 34、给任职日期超过 30 年的员工加薪 10%.
 ```sql
 update emp set sal = sal * 1.1 where timestampdiff(YEAR, hiredate, now()) > 30;
 ```