mysql-子查询

含义

嵌套在其他语句内部的select语句，称为子查询或内查询。
内部嵌套其他select语句的查询，称为外查询或主查询。

分类

按自查询出现的位置分类：

• -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 ：支持标量子查询和列子查询
• exists后面（相关子查询）：支持表子查询

按结果集的行列数不同：

• 标量子查询（结果集只有一行一列）又称单行子查询
• 列子查询（结果集为多行一列）又称多行子查询
• 行子查询（结果集为多行多列）
• 表子查询（结果集一般为多行多列）又称：嵌套子查询

标量子查询

又称单行子查询，出现在where或having后面
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，一般搭配单行操作符使用，如> < <>
④列子查询，一般搭配着多行操作符使用 如：in、any all
⑤子查询的执行优先于主查询，主查询的条件用到了子查询的结果。

案例1：谁的工资比Abel高
①首先先查询下Abel的工资

SELECT salary 
FROM employees
WHERE last_name='Abel';

②在查询员工的信息，要求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143号员工相同，salary比143号员工多的姓名、job_id,和工资
①首先先去查询，143号员工的job_id

SELECT job_id
FROM employees
WHERE employee_id=143;

②在接着查询143号员工的工资

SELECT salary
FROM employees
WHERE employee_id=143;

③最后查询员工的last_name,job_id,salary要求：job_id=①结果，salary>②结果

SELECT last_name,job_id,salary
FROM employees
WHERE job_id=(
	SELECT job_id
	FROM employees
	WHERE employee_id=143
)
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,salary
①首先先查询公司的最低工资

select min(salary)
from employees;

②其次查询last_name,job_id,salary，要求salary=①结果

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号部门的最低工资的部门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)
from employees
group by department_id;

③在②的基础上筛选，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
);

列子查询

又称多行子查询，需搭配多行比较操作符使用

操作符	含义
IN/NOT IN	等于列表中的任意一个
ANY|SOME	和子查询返回的某一个值比较
ALL	和子查询返回的所有值比较

案例1：返回location_id是1400或1700的部门中的所有员工姓名
①先查询部门编号为1400或1700下的location_id都有哪些组成一个表（一列多行）

select distinct department_id
from departments
where location_id in (1400,1700);

②查询员工姓名，要求部门编号是①列表中的任意一个

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

案例2：返回其他部门中比job_id为‘IT_PROG’部门任一工资低的员工的：工号、姓名、job_id,以及salary
①首先先查询job_id=‘IT_PROG’的所有员工的工资

SELECT DISTINCT salary
FROM employees
WHERE job_id='IT_PROG';

②查询员工的工号、姓名、job_id,以及salary，要求salary<①结果中的任意一个

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';

或者：小于任意一个值，可以改成小于最大值

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’的所有工资都低的员工的工号，姓名、job_id，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';

或者是

SELECT employee_id,last_name,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

结果集为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
);

使用行子查询写：

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

子查询放在select后面

仅仅支持标量子查询

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

查询员工号=102的部门名

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

放在from后面

支持表子查询
将子查询结果充当一张表，要求必须给表起别名
案例：查询每个部门的平均工资
①先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，筛选出平均工资between lowest_sal and higest_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 g.`lowest_sal` AND g.`highest_sal`

放在exists后面

又称相关子查询
支持标量子查询、列子查询、行子查询、表子查询
select exists(select employee_id from employees);
结果为1或者0
案例： 查询有员工的部门名

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

使用IN关键字

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

练习题：
1、查询和ZlotKey相同部门的员工姓名和工资
①先查询员工ZlotKey所在的部门

SELECT department_id
FROM employees
WHERE last_name='ZlotKey';

②查询员工姓名、工资，要求此员工的部门=①的结果

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
①查询平均工资

SELECT AVG(salary)
FROM employees;

②查询工资大于①的员工号、姓名、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，工资
①查询各部门的平均工资

select avg(salary),department_id
from employees
group by department_id;

②连接①的结果集和employees表

SELECT employee_id,last_name,salary,e.department_id
FROM employees e
INNER JOIN (
	SELECT AVG(salary) ag,department_id
	FROM employees
	GROUP BY department_id
) ag_dep
ON e.`department_id`=ag_dep.department_id
WHERE salary >ag_dep.ag;

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
①查询姓名中包含字母u的员工所在部门

SELECT DISTINCT department_id
FROM employees
WHERE last_name LIKE('%u%');

②查询员工的员工号、姓名，要求此员工所在的部门号等于①的结果

select employee_id,last_name
from employees 
where department_id in(
	SELECT DISTINCT department_id
	FROM employees
	WHERE last_name LIKE('%u%')
);

5、查询部门的location_id为1700的部门工作的员工的员工号
①查询location_id=1700的部门有哪些

select distinct department_id
from departments
where location_id=1700;

②查询部门号=①的中任意一个的员工号

select employee_id
from employees
where department_id = ANy(
	SELECT DISTINCT department_id
	FROM departments
	WHERE location_id=1700
);

6、查询管理者为King的员工的姓名和工资
①查询姓名为King的员工号

select employee_id
from employees
where last_name ='K_ing';

②查询哪个员工的manager_id等于①的employee_id

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，姓，名
①查询最高工资

SELECT MAX(salary)
FROM employees;

②查询工资=①的员工的姓，名

SELECT CONCAT(first_name,last_name) "姓，名"
FROM employees
WHERE salary=(
	SELECT MAX(salary)
	FROM employees
);

子查询经典案例题
一、查询工资最低的员工信息：last_name,salry
#①先查询最低工资

SELECT MIN(salary)
FROM employees;

#②查询last_name,salary要求salary=①的结果

SELECT last_name,salary 
FROM employees
WHERE salary=(
	SELECT MIN(salary)
	FROM employees
);

二、查询平均工资最低的部门信息
#①各部门的平均工资

SELECT MIN(salary),department_id
FROM employees
GROUP BY department_id;

#②查询①结果上的最低平均工资
SELECT MIN(ag),
FROM (
SELECT MIN(salary) ag,department_id
FROM employees
GROUP BY department_id
)ag_dep;
#③求出最低平均工资的部门编号

SELECT MIN(salary),department_id
FROM employees
GROUP BY department_id
ORDER  BY MIN(salary)
LIMIT 1;

#④查询部门信息

SELECT * FROM 
departments
WHERE department_id =(
	SELECT department_id
	FROM employees
	GROUP BY department_id
	ORDER BY MIN(salary)
	LIMIT 1
);

三、查询平均工资最低的部门信息和该部门的平均工资

SELECT d.*,ag
FROM departments d
INNER JOIN (
	SELECT department_id,AVG(salary)ag
	FROM employees
	GROUP BY department_id
	ORDER BY MIN(salary)
	LIMIT 1
) ag_dep
ON d.`department_id`=ag_dep.department_id;

四、查询平均工资最高的job信息
#①查询每个job的平均工资

SELECT AVG(salary),job_id
FROM employees
GROUP BY job_id;

#②查询job_id最高的平均工资

SELECT AVG(salary),job_id
FROM employees
GROUP BY job_id
ORDER BY AVG(salary) DESC
LIMIT 1;

#③查询job信息

SELECT *
FROM jobs
WHERE job_id=(
	SELECT job_id
	FROM employees
	GROUP BY job_id
	ORDER BY AVG(salary) DESC
	LIMIT 1
);

五、查询平均工资高于公司平均工资的部门有哪些
#①先去查询公司的总的平均工资

SELECT AVG(salary) FROM employees;

#②查询每个部门的平均工资

SELECT AVG(salary),department_id
FROM employees
GROUP BY department_id;

#③ 筛选②结果集，要求平均工资>①

SELECT AVG(salary),department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary)>(
	SELECT AVG(salary) FROM employees
);

六、查询公司中所有的manager的详细信息
#①查询所有manager的员工编号

SELECT DISTINCT manager_id
FROM employees;

#②查询详细信息，要求manager_id=①中的任意一个

SELECT *
FROM employees
WHERE employee_id= ANY(
	SELECT DISTINCT manager_id
	FROM employees
);

七、各部门中最高工资中最低的那个部门的最低工资是多少
#①查询各部门的最高工资中最低的部门编号

SELECT DISTINCT department_id
FROM employees;
GROUP BY department_id
ORDER BY MAX(salary)
LIMIT 1;

#②查询①结果的那个部门的最低工资

SELECT MIN(salary),department_id
FROM employees;
WHERE department_id=(
	SELECT DISTINCT department_id
	FROM employees;
	GROUP BY department_id
	ORDER BY MAX(salary)
	LIMIT 1
);

八、查询平均工资最高的部门的manager的详细信息：last_name,department_id,salary,email
#①查询平均工资最高的部门编号

SELECT department_id
FROM employees
GROUP BY department_id
ORDER BY AVG(salary)DESC
LIMIT 1;

#②将employees 与 departments表连接，筛选条件=①

SELECT last_name,e.department_id,salary,email
FROM employees e
INNER JOIN departments d
ON e.`employee_id`=d.`manager_id`
WHERE d.department_id =(
	SELECT department_id
	FROM employees
	GROUP BY department_id
	ORDER BY AVG(salary)DESC
	LIMIT 1
);