1. 含义：
  2. 出现在其他语句中的select语句，称为子查询或内查询
  3. 外部的查询语句，称为主查询或外查询
  4. 分类：
  5. 按子查询出现的位置：
  6.     select后面：
  7.             仅仅支持标量子查询
  9.     from后面：
  10.              支持表子查询
  12.     where或having后面：★
  13.       标量子查询（单行） √
  14.       列子查询  （多行） √
  16.             行子查询
  18.     exists后面（相关子查询）:
  19.             表子查询
  21. 按结果集的行列数不同：
  22.     标量子查询（结果集只有一行一列）
  23.     列子查询（结果集只有一列多行）
  24.     行子查询（结果集有一行多列）
  25.     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）
2、列子查询（多行子查询）
3、行子查询（多列多行,用的较少）

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?

#①查询Abel的工资
SELECT salary
FROM employees
WHERE last_name = 'Abel'
#②查询员工的信息，满足 salary>①结果
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
#①查询141号员工的job_id
SELECT job_id
FROM employees
WHERE employee_id = 141
#②查询143号员工的salary
SELECT salary
FROM employees
WHERE employee_id = 143
#③查询员工的姓名，job_id 和工资，要求job_id=①并且salary>②
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
#①查询公司的 最低工资
SELECT MIN(salary)
FROM employees
#②查询last_name,job_id和salary，要求salary=①
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
#①查询50号部门的最低工资
SELECT  MIN(salary)
FROM employees
WHERE department_id = 50
#②查询每个部门的最低工资
SELECT MIN(salary),department_id
FROM employees
GROUP BY department_id
#③ 在②基础上筛选，满足min(salary)>①
SELECT MIN(salary),department_id
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
SELECT MIN(salary),department_id
FROM employees
GROUP BY department_id
HAVING MIN(salary)>(
    SELECT  salary
    FROM employees
    WHERE department_id = 250
);

2.列子查询（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

#①查询location_id是1400或1700的部门编号
SELECT DISTINCT department_id
FROM departments
WHERE location_id IN(1400,1700)
#②查询员工姓名，要求部门号是①列表中的某一个
SELECT last_name
FROM employees
WHERE department_id  <>ALL(
    SELECT DISTINCT department_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
#①查询job_id为‘IT_PROG’部门任一工资
SELECT DISTINCT salary
FROM employees
WHERE job_id = 'IT_PROG'
#②查询员工号、姓名、job_id 以及salary，salary<(①)的任意一个
SELECT last_name,employee_id,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 last_name,employee_id,job_id,salary
FROM employees
WHERE salary<(
    SELECT 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 last_name,employee_id,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 last_name,employee_id,job_id,salary
FROM employees
WHERE salary<(
    SELECT 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

SELECT *  
FROM employees
WHERE (employee_id,salary)=(
    SELECT MIN(employee_id),MAX(salary)
    FROM employees
);

***************上下等效****************************

#①查询最小的员工编号
SELECT MIN(employee_id)
FROM employees
#②查询最高工资
SELECT MAX(salary)
FROM employees
#③查询员工信息
SELECT *
FROM employees
WHERE employee_id=(
    SELECT MIN(employee_id)
    FROM employees
)AND salary=(
    SELECT 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`
) 个数
FROM departments d;


#案例2：查询员工号=102的部门名

SELECT (
    SELECT department_name,e.department_id
    FROM departments d
    INNER JOIN employees e
    ON d.department_id=e.department_id
    WHERE e.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
#①查询每个部门的平均工资
SELECT AVG(salary),department_id
FROM employees
GROUP BY department_id
SELECT * FROM job_grades;
#②连接①的结果集和job_grades表，筛选条件平均工资 between lowest_sal and highest_sal
SELECT  ag_dep.*,g.`grade_level`
FROM (
    SELECT AVG(salary) ag,department_id
    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=300000);
#案例1：查询有员工的部门名
#in
SELECT department_name
FROM departments d
WHERE d.`department_id` IN(
    SELECT department_id
    FROM employees
)
#exists
SELECT department_name
FROM departments d
WHERE EXISTS(
    SELECT *
    FROM employees e
    WHERE d.`department_id`=e.`department_id`
);
#案例2：查询没有女朋友的男神信息
#in
SELECT bo.*
FROM boys bo
WHERE bo.id NOT IN(
    SELECT boyfriend_id
    FROM beauty
)
#exists
SELECT bo.*
FROM boys bo
WHERE NOT EXISTS(
    SELECT boyfriend_id
    FROM beauty b
    WHERE bo.`id`=b.`boyfriend_id`
);