初学MySQL第七天_七天入门mysql

作者：笔触狂放9 | 2024-05-04 21:55:09

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七天入门mysql

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（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）
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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结果
 SELECT *
 FROM employees
 WHERE salary>(
 		SELECT salary
 		FROM employees
 		WHERE last_name = 'Abel'
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);

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
 
 );
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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
 );
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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
 );
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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
	);
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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)
 );
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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';
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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';
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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
 );
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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;
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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;
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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 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;
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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);
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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
)
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#exists

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

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