MySQL子查询

子查询重点：where和having后面

术语：

• 标量子查询（一行一列，也就是一个值）
• 列子查询（一列多行）
• 行子查询（一行多列）

特点：

• 子查询一般放在小括号内
• 子查询一般放在条件的右侧
• 标量子查询一般搭配着单行操作符使用
  单行操作符：> < >= <= = <>
• 列子查询，一般搭配着多行操作符使用
  多行操作符：in,any/some,all

标量子查询
案例1：谁的工资比Abel高

SELECT *
FROM employees
WHERE salary>(
	SELECT salary
	FROM employees e
	WHERE e.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 salary>(
	SELECT salary 
	FROM employees
	WHERE employee_id=143
)
AND job_id=(
	SELECT job_id
	FROM employees
	WHERE employee_id=141
);

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和其最低工资

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
);

列子查询
案例1：返回location_id是1400或者1700的部门中的所有员工姓名

SELECT last_name
FROM employees
WHERE department_id IN (
	SELECT DISTINCT department_id 
	from departments
	WHERE location_id IN (1400,1700)
);

IN 可以替换为：=ANY

SELECT last_name
FROM employees
WHERE department_id =ANY (
	SELECT DISTINCT department_id 
	from departments
	WHERE location_id IN (1400,1700)
);

ＮOT IN可以替换成<>ALL

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

SELECT employee_id,job_id,salary
FROM employees
WHERE salary'IT_PROG';

比任一工资低，可以替换为比最高的工资低

SELECT 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,job_id,salary
FROM employees
WHERE salary'IT_PROG';

同样，比所有的工资都低，可以替换成比最低工资的都低

SELECT employee_id,job_id,salary
FROM employees
WHERE salary<(
	SELECT min(salary)
	FROM employees
	WHERE job_id='IT_PROG'
)
AND job_id<>'IT_PROG';

行子查询（用的较少）
案例：查询员工编号最小并且工资最高的员工信息
一般的思想：

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

行子查询思想：

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

位于select后面的子查询（仅仅支持标量子查询）

案例1：查询每个部门的员工个个数

SELECT d.*,(
	SELECT COUNT(*)
	FROM employees e
	WHERE e.department_id=d.department_id
)
from departments d;

案例2：查询员工号为102的部门名

SELECT(
	SELECT department_name 
	FROM departments d
	LEFT JOIN employees e
	ON d.department_id=e.department_id
	WHERE e.employee_id=102
) 部门名;

位于from后面的子查询

相当于把子查询的结果集充当于一个表来使用,要求必须取别名

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 department_id,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;

位于exists后面的子查询(相关子查询)
exists(完整的查询语句)
结果为1表示存在；0表示不存在
类似于true和false

案例1：查询有员工的部门名

SELECT department_name
FROM departments d
WHERE EXISTS(
	SELECT *
	FROM employees e
	WHERE e.department_id=d.department_id
);

用IN来代替

SELECT department_name
FROM departments d
WHERE d.department_id IN(
	SELECT department_id 
	FROM employees e
);

案例2：查询没有女朋友的男神信息

SELECT bo.*
FROM boys bo
WHERE bo.id NOT IN(
SELECT boyfriend_id
from beauty
);

用exists来代替

SELECT bo.*
FROM boys bo
WHERE NOT EXISTS(
	SELECT * 
	FROM beauty b
	WHERE b.boyfriend_id=bo.id
);