MySQL学习8-子查询（非常重要）

目录

SQL 子查询示例

子查询概念

子查询出现位置

WHERE 子句中的子查询关键字

子查询结果集类型

示例查询

1. 查询工资高于JONES的员工

2. 查询与SCOTT同部门的员工

3. 查询工资高于30号部门所有人的员工

4. 查询工作和工资与MARTIN完全相同的员工

5. 查询有2个以上直接下属的领导信息

6. 查询员工编号为7788的员工信息

7. 创建dept2表并插入数据

8. 查询员工及其经理信息

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学生表：stu

CREATE TABLE stu (         sid   CHAR(6),         sname               VARCHAR(50),         age           INT,         gender     VARCHAR(50) );

INSERT INTO stu VALUES('S_1001', 'liuYi', 35, 'male'); INSERT INTO stu VALUES('S_1002', 'chenEr', 15, 'female'); INSERT INTO stu VALUES('S_1003', 'zhangSan', 95, 'male'); INSERT INTO stu VALUES('S_1004', 'liSi', 65, 'female'); INSERT INTO stu VALUES('S_1005', 'wangWu', 55, 'male'); INSERT INTO stu VALUES('S_1006', 'zhaoLiu', 75, 'female'); INSERT INTO stu VALUES('S_1007', 'sunQi', 25, 'male'); INSERT INTO stu VALUES('S_1008', 'zhouBa', 45, 'female'); INSERT INTO stu VALUES('S_1009', 'wuJiu', 85, 'male'); INSERT INTO stu VALUES('S_1010', 'zhengShi', 5, 'female'); INSERT INTO stu VALUES('S_1011', 'xxx', NULL, NULL);

雇员表：emp

CREATE TABLE emp(         empno              INT,         ename              VARCHAR(50),         job           VARCHAR(50),         mgr          INT,         hiredate   DATE,         sal            DECIMAL(7,2),         comm               decimal(7,2),         deptno             INT ) ;

INSERT INTO emp values(7369,'SMITH','CLERK',7902,'1980-12-17',800,NULL,20); INSERT INTO emp values(7499,'ALLEN','SALESMAN',7698,'1981-02-20',1600,300,30); INSERT INTO emp values(7521,'WARD','SALESMAN',7698,'1981-02-22',1250,500,30); INSERT INTO emp values(7566,'JONES','MANAGER',7839,'1981-04-02',2975,NULL,20); INSERT INTO emp values(7654,'MARTIN','SALESMAN',7698,'1981-09-28',1250,1400,30); INSERT INTO emp values(7698,'BLAKE','MANAGER',7839,'1981-05-01',2850,NULL,30); INSERT INTO emp values(7782,'CLARK','MANAGER',7839,'1981-06-09',2450,NULL,10); INSERT INTO emp values(7788,'SCOTT','ANALYST',7566,'1987-04-19',3000,NULL,20); INSERT INTO emp values(7839,'KING','PRESIDENT',NULL,'1981-11-17',5000,NULL,10); INSERT INTO emp values(7844,'TURNER','SALESMAN',7698,'1981-09-08',1500,0,30); INSERT INTO emp values(7876,'ADAMS','CLERK',7788,'1987-05-23',1100,NULL,20); INSERT INTO emp values(7900,'JAMES','CLERK',7698,'1981-12-03',950,NULL,30); INSERT INTO emp values(7902,'FORD','ANALYST',7566,'1981-12-03',3000,NULL,20); INSERT INTO emp values(7934,'MILLER','CLERK',7782,'1982-01-23',1300,NULL,10);

部门表：dept

CREATE TABLE dept(         deptno             INT,         dname              varchar(14),         loc            varchar(13) );

INSERT INTO dept values(10, 'ACCOUNTING', 'NEW YORK'); INSERT INTO dept values(20, 'RESEARCH', 'DALLAS'); INSERT INTO dept values(30, 'SALES', 'CHICAGO'); INSERT INTO dept values(40, 'OPERATIONS', 'BOSTON');

SQL 子查询示例

子查询概念

子查询是指在一个 SELECT 语句中嵌套另一个完整的 SELECT 语句。当一条 SQL 语句包含两个或更多 SELECT 语句时，就构成了子查询。

子查询出现位置

• WHERE 子句后：作为查询条件的一部分
• FROM 子句后：作为临时表使用

WHERE 子句中的子查询关键字

• ANY
• ALL

子查询结果集类型

• 单行单列（用于条件判断）
• 单行多列（用于条件判断）
• 多行单列（用于条件判断）
• 多行多列（作为临时表）

示例查询

1. 查询工资高于JONES的员工

-- 单步查询
SELECT * FROM emp WHERE sal > (SELECT sal FROM emp WHERE ename = 'JONES');

2. 查询与SCOTT同部门的员工

-- 单步查询
SELECT * FROM emp WHERE deptno = (SELECT deptno FROM emp WHERE ename = 'SCOTT');

3. 查询工资高于30号部门所有人的员工

-- 方法1: 使用MAX函数
SELECT * FROM emp WHERE sal > (SELECT MAX(sal) FROM emp WHERE deptno = 30);

-- 方法2: 使用ALL关键字
SELECT * FROM emp WHERE sal > ALL(SELECT sal FROM emp WHERE deptno = 30);

4. 查询工作和工资与MARTIN完全相同的员工

-- 方法1: 分开条件
SELECT * FROM emp 
WHERE job = (SELECT job FROM emp WHERE ename = 'MARTIN') 
AND sal = (SELECT sal FROM emp WHERE ename = 'MARTIN');

-- 方法2: 使用IN子句
SELECT * FROM emp WHERE (job, sal) IN (SELECT job, sal FROM emp WHERE ename = 'MARTIN');

5. 查询有2个以上直接下属的领导信息

SELECT * FROM emp 
WHERE empno IN (SELECT mgr FROM emp GROUP BY mgr HAVING COUNT(mgr) > 2);

6. 查询员工编号为7788的员工信息

-- 方法1: 常规连接查询
SELECT e.ename, e.sal, d.dname, d.loc 
FROM emp e, dept d 
WHERE e.deptno = d.deptno AND e.empno = 7788;

-- 方法2: 使用子查询
SELECT e.ename, e.sal, d.dname, d.loc 
FROM emp e, (SELECT dname, loc, deptno FROM dept) d 
WHERE e.deptno = d.deptno AND e.empno = 7788;

7. 创建dept2表并插入数据

CREATE TABLE `dept2` (
  `deptno` INT DEFAULT NULL,
  `dname` VARCHAR(14) DEFAULT NULL,
  `loc` VARCHAR(13) DEFAULT NULL,
  `desc` VARCHAR(30) DEFAULT NULL
) ENGINE=INNODB DEFAULT CHARSET=utf8mb3;

INSERT INTO dept2 VALUES(10, 'ACCOUNTING', 'NEW YORK', 'good');
INSERT INTO dept2 VALUES(20, 'RESEARCH', 'DALLAS', 'gooood');
INSERT INTO dept2 VALUES(30, 'SALES', 'CHICAGO', 'goodooooo');
INSERT INTO dept2 VALUES(40, 'OPERATIONS', 'BOSTON', 'goooooooood');

8. 查询员工及其经理信息

SELECT e1.empno, e1.ename, e2.mgr, e2.ename 
FROM emp e1, emp e2 
WHERE e1.mgr = e2.empno AND e1.empno = 7369;