lyt经典版MySQL基础——进阶7：子查询

#进阶7：子查询
/*
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（√）
        列子查询  （√）
        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（结果集有多行一列）
    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）
2.列子查询（多行子查询）
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？
#(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)的结果
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 department_id,MIN(salary) '最低工资'
FROM employees
GROUP BY department_id
#(3)在(2)的基础上筛选，满足min(salary)>(1)
SELECT department_id,MIN(salary) '最低工资'
FROM employees
GROUP BY department_id
HAVING 最低工资>(
    SELECT MIN(salary)
    FROM employees
    WHERE department_id='50'
);

#非法使用标量子查询——子查询的结果不是一行一列
SELECT department_id,MIN(salary) '最低工资'
FROM employees
GROUP BY department_id
HAVING 最低工资>(
    SELECT salary
    FROM employees
    WHERE department_id='50'
);


#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 IN(
    SELECT DISTINCT department_id
    FROM departments
    WHERE location_id IN(1400,1700)
);
#或
SELECT last_name
FROM employees 
WHERE department_id =ANY(
    SELECT DISTINCT department_id
    FROM departments
    WHERE location_id IN(1400,1700)
);
#案例2：返回其他工种中比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'工种任一工资
SELECT salary
FROM employees
WHERE job_id='IT_PROG'
#(2)查询员工号、姓名、job_id以及salary，salary<(1)中的任意一个
SELECT employee_id,last_name,job_id,salary
FROM employees
WHERE salary<ANY(
    SELECT 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 MAX(salary)
    FROM employees
    WHERE job_id='IT_PROG'
)AND job_id <>'IT_PROG';
#案例3：返回其他工种中比job_id为'IT_PROG'工种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
#(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
);
#或
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：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 
    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
#(2)连接(1)的结果集和job_grades表，筛选条件平均工资
SELECT ag_dep.*,grade_level
 FROM (
    SELECT AVG(salary) ag,department_id
    FROM employees
    GROUP BY department_id
) ag_dep
INNER JOIN job_grades 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
SELECT department_name FROM departments
WHERE department_id IN(
SELECT department_id FROM employees
WHERE employee_id IS NOT NULL
);
#exists
SELECT department_name FROM departments d
WHERE EXISTS (
    SELECT employee_id 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 * FROM boys 
WHERE boys.id NOT IN (
    SELECT boyfriend_id FROM beauty
);
#exists
SELECT * FROM boys bo
WHERE NOT EXISTS(
SELECT boyfriend_id FROM beauty b
WHERE bo.id=b.boyfriend_id
);