MySQL ---- SQL语句学习笔记三_sql 计算判断-优快云博客

本文介绍SQL子查询的应用场景，包括WHERE、SELECT、HAVING、ON及FROM子句中的子查询用法，并通过实例展示了如何找出每个部门工资最高的员工。

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实例用表

emp表

id	name	salary	depId
1	Joe	70000	1
2	Henry	80000	2
3	Sam	60000	2
4	Max	90000	1

CREATE TABLE emp(id INT,name char(255),salary int, depId int);

INSERT INTO emp VALUES(1,'Joe',70000,1),(2,'Henry',80000,2),(3,'Sam',60000,2),(4,'Max',90000,1);

dep表

id	name
1	IT
2	Sales

CREATE TABLE dep(id INT,name char(255));

INSERT INTO dep VALUES(1,'IT'),(2,'Sales');

1.WHERE 里边的子查询

子查询如果在 where 或 select 中时，相当于【子查询】先计算结果再判断！
“=” 的子查询返回必须唯一，否则就会报错！
IN 的子查询可以返回一列的结果！

1.1 找出每个部门工资最高的员工！

SELECT
	dep. NAME AS '部门',
	p2. NAME AS '员工',
	p2.salary
FROM
	emp p2
JOIN dep ON dep.id = p2.depId
WHERE
	p2.salary IN (
		SELECT
			MAX(salary) AS salary
		FROM
			emp p1
		GROUP BY
			p1.depId
	)

2.SELECT 里边的子查询

子查询返回必须唯一！否则就会报错！

2.1 找出全部员工中工资最高的，添加max行！

SELECT
	dep. NAME,
	p2. NAME,
	p2.salary,
	(
		SELECT
			MAX(salary) AS salary
		FROM
			emp p1
	) AS max
FROM
	emp p2
JOIN dep ON dep.id = p2.depId

3.having 里边的子查询

不常见，基本不用！

4.ON 里边的子查询

不常见，基本不用！

5.FROM 里边的子查询

在 FROM 里边的子查询必须要有别名！

5.1 找出每个部门工资最高的员工！

SELECT
	dep. NAME AS '部门',
	p2. NAME AS '员工',
	p2.salary
FROM
	emp p2
JOIN dep ON dep.id = p2.depId
JOIN (
	SELECT
		MAX(salary) AS salary,
		depId
	FROM
		emp p1
	GROUP BY
		p1.depId
) p3 ON p2.depId = p3.depId
AND p2.salary = p3.salary

6.半连接查询

子查询中使用了父查询的信息！

6.1 找出每个部门工资最高的员工！

SELECT
	dep. NAME AS '部门',
	p1. NAME AS '员工',
	p1.salary
FROM
	emp p1
JOIN dep ON dep.id = p1.depId
WHERE
	p1.salary = (
		SELECT
			MAX(salary) AS max
		FROM
			emp p2
		WHERE
			p2.depId = p1.depId
	)