写在前面

• 若子查询结构复杂，则建议从里往外写；若是相关子查询，通常从外往里写
• 在select中，除了group by和limit之外，其他位置都可以声明子查询
• 子查询的结果被主查询使用
• 注意事项：
  • 子查询要包含在括号内
  • 将子查询放在比较条件的右侧
  • 单行操作符对应单行子查询，多行操作符对应多行子查询
  • Mysql中聚合函数不能嵌套，Oracle中聚合函数可以嵌套
  • from 后面的子查询为一张虚拟表，必须有别名

一、子查询的分类

• 角度1：单行子查询 vs 多行子查询
  • 结果集包含的条目数：单行或多行
• 角度2：相关子查询 vs 不相关子查询
  • 子查询是否被执行多次。
    • 相关子查询的需求：查询工资大于本部门平均工资的员工信息。查询的数据与返回的结果集相关
    • 不相关子查询的需求：查询工资大于本公司平均工资的员工信息。

二、单行子查询 vs 多行子查询

2.1 单行子查询

• 单行操作符：=、>、>=、<、<=、<>（不相等）

# 练习：查询工资大于149号员工工资的员工的信息
SELECT last_name,salary
FROM employees
WHERE salary > (
				SELECT salary
				FROM employees
				WHERE employee_id = 149
			   );

• HAVING 后的子查询

# 练习：查询最低工资大于110号部门最低工资的部门id和其最低工资
SELECT department_id,MIN(salary)
FROM employees
WHERE department_id IS NOT NULL
GROUP BY department_id
HAVING MIN(salary) > (
						SELECT MIN(salary)
						FROM employees
						WHERE department_id = 110
					 );

• ** CASE中的子查询**

# 练习：显示员工的employee_id,last_name和location。
	# 其中，若员工department_id与location_id为1800的department_id相同
	# 则location为’Canada’，其余则为’USA’。
SELECT employee_id,last_name,(CASE department_id WHEN (
														SELECT department_id 
														FROM departments 
														WHERE location_id = 1800) 
						      THEN 'Canada'
						      ELSE 'USA'
						      END
						      ) 'location'
FROM employees;

• 注意事项：
  1、子查询中的空值问题。若内查询结果为空，整体结果为空
  2、非法使用子查询，例：多行子查询使用单行比较符

2.2 多行子查询

• 多行操作符：in、any、all、some(同any)

# 练习：查询与141号或174号员工的manager_id和department_id相同的其他员工的employee_id，manager_id，department_id
SELECT employee_id,manager_id,department_id
FROM employees
WHERE manager_id IN (
						SELECT manager_id
						FROM employees
						WHERE employee_id IN (141,174)
					)
AND department_id IN (
						SELECT department_id
						FROM employees
						WHERE employee_id IN (141,174)
					 )
AND employee_id <> 141;

三、相关子查询 vs 不相关子查询

3.1 相关子查询

• 子查询中用到了外部的表

# 练习：查询员工中工资大于本部门平均工资的员工的last_name,salary和其department_id
SELECT last_name,salary,department_id
FROM employees e1
WHERE salary > (
		SELECT AVG(salary) avg_sal
		FROM employees e2
		WHERE department_id = e1.department_id
		);

# 方法2：在from中声明
SELECT e.last_name,e.salary,e.department_id
FROM employees e JOIN (
						SELECT AVG(salary) avg_sal,department_id
						FROM employees e2
						GROUP BY department_id
					  ) dept_avg_sal
ON e.department_id = dept_avg_sal.department_id
WHERE e.salary > avg_sal;

• 在ORDER BY 中使用子查询

# 练习：查询员工的id,salary,按照department_name 排序
SELECT employee_id,salary
FROM employees e
ORDER BY (
		 SELECT department_name
		 FROM departments d
		 WHERE e.`department_id` = d.`department_id`
		 );

3.2 不相关子查询

# 练习：查询平均工资最低的部门id
SELECT department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary) <= ALL(
		SELECT AVG(salary)
		FROM employees
		GROUP BY department_id
		);

3.3 EXISTS 与 NOT EXISTS关键字

• EXISTS：如果存在某种条件，则返回TRUE，否则返回FALSE

# 练习12：查询公司管理者的employee_id，last_name，job_id，department_id信息
# 子查询
SELECT employee_id,last_name,job_id,department_id
FROM employees
WHERE employee_id IN(
				    SELECT DISTINCT(manager_id)
				    FROM employees
				    );

# 使用exists
SELECT employee_id,last_name,job_id,department_id
FROM employees e1
WHERE EXISTS (
		     SELECT *
		     FROM employees e2
		     WHERE e1.employee_id = e2.manager_id
		     );

• NOT EXISTS：如果不存在某种条件，则返回TRUE，否则返回FALSE

# 练习：查询departments表中，不存在于employees表中的部门的department_id和department_name
# 左外连接
SELECT d.department_id,d.department_name
FROM departments d LEFT JOIN employees e
ON d.department_id = e.department_id
WHERE e.department_id IS NULL; 

# 子查询
SELECT department_id,department_name
FROM departments d
WHERE NOT EXISTS(
				SELECT *
				FROM employees e
				WHERE d.department_id = e.department_id
				);

四、重要习题

# 查询平均工资最低的部门信息和该部门的平均工资（相关子查询）
SELECT d.*,AVG(salary)
FROM departments d JOIN employees e
ON e.department_id = d.department_id
GROUP BY e.department_id
HAVING e.department_id = (
							SELECT department_id
							FROM employees
							GROUP BY department_id
							HAVING AVG(salary) <= ALL(
												      SELECT AVG(salary)
												      FROM employees
												      GROUP BY department_id
												     )
						  );

# 查询各个部门中 最高工资中最低的那个部门的 最低工资是多少?
SELECT MIN(salary)
FROM employees
WHERE department_id = (
				      SELECT department_id
				      FROM employees
				      WHERE department_id IS NOT NULL
				      GROUP BY department_id
				      ORDER BY MAX(salary) ASC
				      LIMIT 1
				      );

# 查询每个部门下的部门人数大于 5 的部门名称（相关子查询）
SELECT department_name
FROM departments d
WHERE 5 < (
		    SELECT COUNT(*)
		    FROM employees e
		    GROUP BY department_id
		    HAVING d.department_id = e.department_id
		    );

# 查询每个国家下的部门个数大于 2 的国家编号（相关子查询）
SELECT country_id
FROM locations l
WHERE 2 < (
		   SELECT COUNT(*)
		   FROM departments d
		   WHERE l.location_id = d.location_id
		  );

参考：尚硅谷-宋红康老师的视频及课件。