MySQL_04SQL子查询
MySQL_04SQL子查询
同步脑图：https://www.processon.com/view/60f5150c1efad41bbeab0997#map
1.什么是子查询
select语句中嵌套select语句，被嵌套的select语句被称为子查询
2.在什么地方可以使用子查询
- select..(select)
- from...(select)
- where(select)
3.在where后面使用
3.1案例
- 找出比最低工资高的员工姓名和工资
3.2实现
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第一步
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查询出最低工资
mysql> select min(sal) from emp; +----------+ | min(sal) | +----------+ | 800.00 | +----------+ 1 row in set (0.01 sec)
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第二步
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找出大于800的员工姓名和工资
mysql> select ename,sal from emp where sal > 800; +--------+---------+ | 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 | +--------+---------+ 13 rows in set (0.01 sec)
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第三步
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合并
mysql> select ename,sal from emp where sal > (select min(sal) from emp); +--------+---------+ | 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 | +--------+---------+ 13 rows in set (0.02 sec)
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4.在from后面使用
4.1案例
- 找出每个岗位的平均工资的薪资等级
4.2特点
- 将子查询的结果当作一张临时表
4.3涉及到的表
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emp表
+-------+--------+-----------+------+------------+---------+---------+--------+ | EMPNO | ENAME | JOB | MGR | HIREDATE | SAL | COMM | DEPTNO | +-------+--------+-----------+------+------------+---------+---------+--------+ | 7369 | SMITH | CLERK | 7902 | 1980-12-17 | 800.00 | NULL | 20 | | 7499 | ALLEN | SALESMAN | 7698 | 1981-02-20 | 1600.00 | 300.00 | 30 | | 7521 | WARD | SALESMAN | 7698 | 1981-02-22 | 1250.00 | 500.00 | 30 | | 7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975.00 | NULL | 20 | | 7654 | MARTIN | SALESMAN | 7698 | 1981-09-28 | 1250.00 | 1400.00 | 30 | | 7698 | BLAKE | MANAGER | 7839 | 1981-05-01 | 2850.00 | NULL | 30 | | 7782 | CLARK | MANAGER | 7839 | 1981-06-09 | 2450.00 | NULL | 10 | | 7788 | SCOTT | ANALYST | 7566 | 1987-04-19 | 3000.00 | NULL | 20 | | 7839 | KING | PRESIDENT | NULL | 1981-11-17 | 5000.00 | NULL | 10 | | 7844 | TURNER | SALESMAN | 7698 | 1981-09-08 | 1500.00 | 0.00 | 30 | | 7876 | ADAMS | CLERK | 7788 | 1987-05-23 | 1100.00 | NULL | 20 | | 7900 | JAMES | CLERK | 7698 | 1981-12-03 | 950.00 | NULL | 30 | | 7902 | FORD | ANALYST | 7566 | 1981-12-03 | 3000.00 | NULL | 20 | | 7934 | MILLER | CLERK | 7782 | 1982-01-23 | 1300.00 | NULL | 10 | +-------+--------+-----------+------+------------+---------+---------+--------+ 14 rows in set (0.00 sec) -
salgrade表
+-------+-------+-------+ | GRADE | LOSAL | HISAL | +-------+-------+-------+ | 1 | 700 | 1200 | | 2 | 1201 | 1400 | | 3 | 1401 | 2000 | | 4 | 2001 | 3000 | | 5 | 3001 | 9999 | +-------+-------+-------+ 5 rows in set (0.05 sec)
4.4实现
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第一步
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找出每个岗位的平均工资
mysql> select job,avg(sal) from emp group by job; +-----------+-------------+ | job | avg(sal) | +-----------+-------------+ | ANALYST | 3000.000000 | | CLERK | 1037.500000 | | MANAGER | 2758.333333 | | PRESIDENT | 5000.000000 | | SALESMAN | 1400.000000 | +-----------+-------------+ 5 rows in set (0.01 sec)
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第二步
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把第一步查询的结果当作一张临时表t，将临时表t和salgrade表进行表连接
mysql> select t.*,s.grade -> from (select job,avg(sal) as avgsal from emp group by job) as t -> join salgrade as s -> on t.avgsal between s.losal and s.hisal; +-----------+-------------+-------+ | job | avgsal | grade | +-----------+-------------+-------+ | CLERK | 1037.500000 | 1 | | SALESMAN | 1400.000000 | 2 | | ANALYST | 3000.000000 | 4 | | MANAGER | 2758.333333 | 4 | | PRESIDENT | 5000.000000 | 5 | +-----------+-------------+-------+ 5 rows in set (0.01 sec)
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5.在select后面使用
5.1案例
- 找出每个员工的部门名称，要求显示员工名、部门名
5.2涉及到的表
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emp表
+-------+--------+-----------+------+------------+---------+---------+--------+ | EMPNO | ENAME | JOB | MGR | HIREDATE | SAL | COMM | DEPTNO | +-------+--------+-----------+------+------------+---------+---------+--------+ | 7369 | SMITH | CLERK | 7902 | 1980-12-17 | 800.00 | NULL | 20 | | 7499 | ALLEN | SALESMAN | 7698 | 1981-02-20 | 1600.00 | 300.00 | 30 | | 7521 | WARD | SALESMAN | 7698 | 1981-02-22 | 1250.00 | 500.00 | 30 | | 7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975.00 | NULL | 20 | | 7654 | MARTIN | SALESMAN | 7698 | 1981-09-28 | 1250.00 | 1400.00 | 30 | | 7698 | BLAKE | MANAGER | 7839 | 1981-05-01 | 2850.00 | NULL | 30 | | 7782 | CLARK | MANAGER | 7839 | 1981-06-09 | 2450.00 | NULL | 10 | | 7788 | SCOTT | ANALYST | 7566 | 1987-04-19 | 3000.00 | NULL | 20 | | 7839 | KING | PRESIDENT | NULL | 1981-11-17 | 5000.00 | NULL | 10 | | 7844 | TURNER | SALESMAN | 7698 | 1981-09-08 | 1500.00 | 0.00 | 30 | | 7876 | ADAMS | CLERK | 7788 | 1987-05-23 | 1100.00 | NULL | 20 | | 7900 | JAMES | CLERK | 7698 | 1981-12-03 | 950.00 | NULL | 30 | | 7902 | FORD | ANALYST | 7566 | 1981-12-03 | 3000.00 | NULL | 20 | | 7934 | MILLER | CLERK | 7782 | 1982-01-23 | 1300.00 | NULL | 10 | +-------+--------+-----------+------+------------+---------+---------+--------+ 14 rows in set (0.00 sec) -
dept表
+--------+------------+----------+ | DEPTNO | DNAME | LOC | +--------+------------+----------+ | 10 | ACCOUNTING | NEW YORK | | 20 | RESEARCH | DALLAS | | 30 | SALES | CHICAGO | | 40 | OPERATIONS | BOSTON | +--------+------------+----------+ 4 rows in set (0.01 sec)
5.3实现
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在select后面完成dname的查询
mysql> select e.ename,(select d.dname from dept d where e.deptno = d.deptno) as dname -> from emp e; +--------+------------+ | ename | dname | +--------+------------+ | SMITH | RESEARCH | | ALLEN | SALES | | WARD | SALES | | JONES | RESEARCH | | MARTIN | SALES | | BLAKE | SALES | | CLARK | ACCOUNTING | | SCOTT | RESEARCH | | KING | ACCOUNTING | | TURNER | SALES | | ADAMS | RESEARCH | | JAMES | SALES | | FORD | RESEARCH | | MILLER | ACCOUNTING | +--------+------------+ 14 rows in set (0.00 sec)
5.4注意
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在select后面的子查询结果一次不能超过一个，不然就报错
mysql> select e.ename,(select d.dname from dept d) as dname from emp e; ERROR 1242 (21000): Subquery returns more than 1 row
