MySQL--基础十（子查询）

快来打我* 2021-08-13 22:37 450阅读 0赞

#### 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（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）

#### 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 IN(
    	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
    );
    
    #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
    
    );

#### 在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,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;

#### 在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;

#### 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`
    
    );

#1. 查询和Zlotkey相同部门的员工姓名和工资
    
    #①查询Zlotkey的部门
    SELECT department_id
    FROM employees
    WHERE last_name = 'Zlotkey'
    
    #②查询部门号=①的姓名和工资
    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。
    
    #①查询平均工资
    SELECT AVG(salary)
    FROM employees
    
    #②查询工资>①的员工号，姓名和工资。
    
    SELECT last_name,employee_id,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
    #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结果集和employees表，进行筛选
    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的员工的部门
    
    SELECT  DISTINCT department_id
    FROM employees
    WHERE last_name LIKE '%u%'
    
    #②查询部门号=①中的任意一个的员工号和姓名
    SELECT last_name,employee_id
    FROM employees
    WHERE department_id IN(
    	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
    
    #①查询location_id为1700的部门
    
    SELECT DISTINCT department_id
    FROM departments 
    WHERE location_id  = 1700
    
    
    #②查询部门号=①中的任意一个的员工号
    SELECT employee_id
    FROM employees
    WHERE department_id =ANY(
    	SELECT DISTINCT department_id
    	FROM departments 
    	WHERE location_id  = 1700
    
    );
    #6.查询管理者是King的员工姓名和工资
    
    #①查询姓名为king的员工编号
    SELECT employee_id
    FROM employees
    WHERE last_name  = 'K_ing'
    
    #②查询哪个员工的manager_id = ①
    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
    
    
    #①查询最高工资
    SELECT MAX(salary)
    FROM employees
    
    #②查询工资=①的姓.名
    
    SELECT CONCAT(first_name,last_name) "姓.名"
    FROM employees
    WHERE salary=(
    	SELECT MAX(salary)
    	FROM employees
    
    );