含义：

出现在其他语句中的SELECT语句，称为子查询或内查询
外部的查询语句，称为主查询或外查询

分类：

按子查询出现的位置：
SELECT后面：仅仅支持标量子查询
 FROM后面：支持表子查询WHERE或HAVING后面：★
 标量子查询（单行） √
 列子查询 （多行） √
 行子查询EXISTS后面（相关子查询）
 表子查询
按结果集的行列数不同：
标量子查询（结果集只有一行一列）
 列子查询（结果集只有一列多行）
 行子查询（结果集有一行多列）
 表子查询（结果集一般为多行多列）

一、where或having后面

1、标量子查询（单行子查询）较多使用
 2、列子查询（多行子查询） 较多使用
 3、行子查询（多列多行）
特点：

①子查询放在小括号内
②子查询一般放在条件的右侧
③标量子查询，一般搭配着单行操作符使用：> < >= <= = <>
列子查询，一般搭配着多行操作符使用：IN、ANY/SOME、ALL
④子查询的执行优先于主查询执行，主查询的条件用到了子查询的结果

1.标量子查询

案例1：谁的工资比 Abel 高?
SELECT
 *
 FROM
employees WHERE salary >
 (SELECT
salary FROM
employees WHERE last_name = ‘Abel’) ;
案例2：返回job_id与141号员工相同，salary比143号员工多的员工 姓名，job_id 和工资
SELECT
last_name,job_id,salary FROM
employees WHERE job_id=(
 SELECT job_id FROM employees WHERE employee_id=141
 )
 AND salary>(
 SELECT salary FROM employees WHERE employee_id=143
 );
案例3：返回公司工资最少的员工的last_name,job_id和salary
SELECT
last_name,job_id,salary FROM
employees WHERE salary=(
 SELECT MIN(salary)
 FROM employees );
案例4：查询最低工资大于50号部门最低工资的部门id和其最低工资
SELECT 
department_id,MIN(salary)
 FROM
employees GROUP BY department_id HAVING MIN(salary)>(
 SELECT MIN(salary)
 FROM employees WHERE department_id=50
 );

非法使用标量子查询

就是子查询结果不为一行一列

2.多行子查询

案例1：返回location_id是1400或1700的部门中的所有员工姓名
SELECT
last_name FROM
employees WHERE department_id IN (
 SELECT DISTINCTdepartment_id FROM departments WHERE location_id IN (1400,1700)
 );
 或
 SELECT
last_name FROM
employees WHERE department_id =ANY (
 SELECT DISTINCTdepartment_id FROM departments WHERE location_id IN (1400,1700)
 );
案例2：返回其它工种中比job_id为‘IT_PROG’工种任一工资低的员工的员工号、姓名、job_id 以及salary
SELECT
employee_id,last_name,job_id,salary FROM
employees WHERE salary < ANY(
 SELECT DISTINCT salary FROM employees WHERE job_id= ‘IT_PROG’
 )
 AND job_id<> ‘IT_PROG’;或
SELECT
employee_id,last_name,job_id,salary FROM
employees WHERE salary < (
 SELECT DISTINCT MAX(salary)
 FROM employees WHERE job_id= ‘IT_PROG’
 )
 AND job_id<> ‘IT_PROG’;
案例3：返回其它部门中比job_id为‘IT_PROG’部门所有工资都低的员工的员工号、姓名、job_id 以及salary
SELECT
employee_id,last_name,job_id,salary FROM
employees WHERE salary < ALL(
 SELECT DISTINCT salary FROM employees WHERE job_id=‘IT_PROG’
 )
 AND job_id<>‘IT_PROG’;或
SELECT
employee_id,last_name,job_id,salary FROM
employees WHERE salary<(
 SELECT DISTINCT MIN(salary)
 FROM employees WHERE job_id=‘IT_PROG’
 )
 AND job_id<>‘IT_PROG’;

3.行子查询（结果集一行多列或多行多列）

案例：查询员工编号最小并且工资最高的员工信息
平常方式：
 SELECT
 *
 FROM
employees WHERE employee_id=(
 SELECT MIN(employee_id)
 FROM employees )
 AND salary=(
 SELECT MAX(salary)
 FROM employees );
 行子查询方式：
 SELECT *
 FROM employees WHERE (employee_id,salary)=(
 SELECT MIN(employee_id),MAX(salary)
 FROM employees );

二、select后面 ：仅仅支持标量子查询

案例1：查询每个部门的员工个数
SELECT d.* , (
 SELECT COUNT(*)
 FROM employees e
 WHERE e.department_id=d.department_id ) 每个部门员工数
 FROM
departments d;
案例2：查询员工号=102的部门名
SELECT (
 SELECT department_name FROM departments d
 INNER JOIN employees e
 ON e.department_id=d.department_id WHERE employee_id=102
 )部门名;

三、from后面 :将子查询结果充当一张表，要求必须起别名

案例：查询每个部门的平均工资的工资等级
SELECT ag_dep.*,g.grade_level FROM (
 SELECT AVG(salary) ag
 FROM employees GROUP BY department_id ) ag_dep
 INNER JOIN job_grades g
 ON ag_dep.ag BETWEEN lowest_sal AND highest_sal;

四、exists后面（相关子查询）

语法：
EXISTS(完整的查询语句)
结果：
1或0

SELECT EXISTS (SELECT employee_id FROM employees WHERE salary=30000);

案例1：查询有员工的部门名
in方式：
SELECT department_name FROM departments d
 WHERE d.department_id IN (
 SELECT department_id FROM employees );
exists方式：
SELECT department_name FROM departments d
 WHERE EXISTS(
 SELECT *
 FROM employees e
 WHERE e.department_id=d.department_id );

练习题

1.查询和Zlotkey相同部门的员工姓名和工资
SELECT
last_name,salary FROM
employees WHERE department_id=(
 SELECT department_id FROM employees WHERE last_name=‘Zlotkey’
 );
2.查询工资比公司平均工资高的员工的员工号，姓名和工资。
SELECT
employee_id,last_name,salary FROM 
employees WHERE salary>(
 SELECT AVG(salary)
 FROM employees );
3.查询各部门中工资比本部门平均工资高的员工的员工号, 姓名和工资
SELECT
employee_id,last_name,salary FROM 
employees e
 INNER JOIN (
 SELECT AVG(salary) ags,department_id FROM employees GROUP BY department_id ) d
 ON e.department_id = d.department_id WHERE salary>d.ags;
4.查询和姓名中包含字母u的员工在相同部门的员工的员工号和姓名
SELECT
employee_id,last_name FROM
employees e
 WHERE department_idIN(
 SELECT DISTINCT department_id FROM employees WHERE last_name LIKE “%u%”
 ) ;
5.查询在部门的location_id为1700的部门工作的员工的员工号
SELECT
employee_id FROM
employees WHERE department_id = ANY (
 SELECT department_id FROM departments WHERE location_id=1700
 );
6.查询管理者是K_ing的员工姓名和工资
SELECT
last_name,salary FROM
employees WHERE manager_id IN(
 SELECT employee_id FROM employees WHERE last_name=“K_ing”
 );
7.查询工资最高的员工的姓名，要求first_name和last_name显示为一列，列名为 姓.名
SELECT
 CONCAT(last_name,first_name) 姓名
 FROM
employees WHERE salary=(
 SELECT MAX(salary)
 FROM employees );