07. 子查询

一、子查询概述

子查询指一个查询语句嵌套在另一个查询语句内部的查询。

2.1.1、子查询的基本语法结构

SELECT 字段名
FROM 表名
WHERE 字段名1 = (
    SELECT 字段名1
    FROM 表名
    WHERE 过滤条件);
  • 子查询（内查询）在主查询之前一次执行完成，子查询的结果被主查询（外查询）使用
  • 子查询要包含在括号内，建议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
SELECT last_name,salary
FROM employees 
WHERE salary > (
	SELECT salary 
	FROM employees 
	WHERE last_name = 'Abel'
);

二、子查询的分类

  • 按内查询的结果返回一条还是多条记录，将子查询分为 单行子查询 、多行子查询。
    • 单行子查询查询的结果返回一条记录
    • 多行子查询查询的结果返回多条记录
  • 按内查询是否被执行多次，将子查询划分为相关(或关联)子查询和不相关(或非关联)子查询。
    • 子查询从数据表中查询了数据结果，如果这个数据结果只执行一次，然后这个数据结果作为主查询的条件进行执行，那么这样的子查询叫做不相关子查询。
    • 子查询从数据表中查询了数据结果，如果子查询需要执行多次，即采用循环的方式，先从外部查询开始，每次都传入子查询进行查询，然后再将结果反馈给外部，这种嵌套的执行方式就称为相关子查询。

2.1、单行子查询

2.1.1、单行比较操作符

操作符 含义
= equal to
> greater than
>= greater than or equal to
< less than
<= less than or equal to
<> not equal to 
-- 查询工资大于149号员工工资的员工的信息
SELECT employee_id,last_name,salary
FROM employees
WHERE salary > (
	SELECT salary
	FROM employees
	WHERE employee_id = 149
);

-- 返回job_id于141号员工相同，salary比143号员工的多的员工姓名，job_id和工资
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
);

-- 返回公司工资最少的员工的last_name,job_id和salary
-- 升序排序，取第一个
SELECT last_name,job_id,salary
FROM employees
ORDER BY salary ASC 
LIMIT 1;
-- 子查询的方法
SELECT last_name,job_id,salary
FROM employees
WHERE salary = (
	SELECT MIN(salary) FROM employees
);

-- 查询与141号或174号员工的manager_id和department_id相同的其它员工的employee_id，manager_id，department_id
SELECT employee_id,manager_id,department_id
FROM employees
WHERE manager_id = (
	SELECT manager_id
	FROM employees
	WHERE employee_id = 141
) AND (
	SELECT department_id
	FROM employees
	WHERE employee_id = 141
) AND employee_id <> 141;
-- 方式2
SELECT employee_id,manager_id,department_id
FROM employees
WHERE (manager_id,department_id) = (
	SELECT manager_id,department_id
	FROM employees
	WHERE employee_id = 141
) AND employee_id <> 141;

2.1.2、HAVING 中的子查询

SELECT   字段名1, 分组函数(字段名)
FROM     表名
GROUP BY 字段名1
HAVING   分组函数1(字段名) = (
    SELECT 分组函数1(字段名)
    FROM   表名
    WHERE  过滤条件);
  • 首先执行子查询。
  • 向主查询中的 HAVING 子句返回结果。
-- 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
GROUP BY department_id
HAVING MIN(salary) > (
	SELECT MIN(salary)
	FROM employees
	WHERE department_id = 110
);

2.1.3、CASE中的子查询

SELECT 字段名
CASE WHEN (
        SELECT 字段名1 
        FROM 表名
        WHERE 过滤条件)           
     THEN 结果1 
     ELSE 结果2 END AS 别名
FROM   表名;

-- 显示员工的employee_id，last_name和location
-- 其中，若员工的department_id与location_id为1800的department_id相同，则location为'Canada'，其余为'USA'
SELECT employee_id,last_name,
CASE WHEN department_id = (
	SELECT department_id
	FROM departments d JOIN locations l
	ON d.location_id = l.location_id
	WHERE l.location_id = 1800
) THEN 'Canada'
     ELSE 'USA' END AS "location"
FROM employees;

2.2、多行子查询

2.2.1、多行子查询的基本语法结构

SELECT 字段名
FROM 表名
WHERE 字段名1 IN (
    SELECT 字段名1
    FROM 表名
    WHERE 过滤条件);
  • 也称为集合比较子查询
  • 内查询返回多行
  • 使用多行比较操作符
SELECT employee_id,last_name
FROM employees
WHERE salary IN (
	SELECT MIN(salary)
	FROM employees
	GROUP BY department_id
);

2.2.2、多行比较操作符

操作符 含义
IN 等于列表中的任意一个
ANY 需要和单行比较操作符一起使用，和子查询返回的某一个值比较
ALL 需要和单行比较操作符一起使用，和子查询返回的所有值比较
SOME 实际上是ANY的别名，作用相同，一般常使用ANY 
-- 返回其它job_id中比job_id为'IT_PROG'部门任一工资低的员工的员工号、姓名、job_id以及salary
SELECT employee_id,last_name,job_id,salary
FROM employees
WHERE salary < ANY(
	SELECT salary 
	FROM employees 
	WHERE job_id = 'IT_PROG'
) AND job_id <> 'IT_PROG';
-- 方式2
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';

-- 返回其它job_id中比job_id为'IT_PROG'部门所有工资低的员工的员工号、姓名、job_id以及salary
SELECT employee_id,last_name,job_id,salary
FROM employees
WHERE salary < ALL(
	SELECT salary 
	FROM employees 
	WHERE job_id = 'IT_PROG'
) AND job_id <> 'IT_PROG';
-- 方式2
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';

-- 查新平均工资最低的部门id
SELECT department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary) = (
	SELECT MIN(avg_sal)
	FROM (
		SELECT AVG(salary) AS "avg_sal"
		FROM employees
		GROUP BY department_id
	) AS t_dept_avg_sal
);
-- 方式2
SELECT department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary) <= ALL(
	SELECT AVG(salary)
	FROM employees
	GROUP BY department_id
);
-- 方式3
SELECT department_id
FROM employees
GROUP BY department_id
ORDER BY AVG(salary) ASC 
LIMIT 1;

2.3、相关子查询

2.3.1、相关子查询的基本语法结构

??如果子查询的执行依赖于外部查询，通常情况下都是因为子查询中的表用到了外部的表，并进行了条件关联，因此每执行一次外部查询，子查询都要重新计算一次，这样的子查询就称之为关联子查询。相关子查询按照一行接一行的顺序执行，主查询的每一行都执行一次子查询。

-- 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
-- 方式1：相关子查询
SELECT last_name,salary,department_id
FROM employees e1
WHERE salary > (
	SELECT AVG(salary)
	FROM employees e2
	WHERE department_id = e1.department_id 
);
-- 方式2：FROM中声明子查询
SELECT e.last_name,e.salary,e.department_id
FROM employees e,(
	SELECT department_id,AVG(salary) AS "avg_sal"
	FROM employees
	GROUP BY department_id
	) AS t_dept_avg_sal
WHERE e.department_id = t_dept_avg_sal.department_id
AND e.salary > t_dept_avg_sal.avg_sal;

-- 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
) ASC;

-- 若emploess表中employee_id与job_history表中employee_id相同的数目不小于2，
-- 输出这些相同id的员工的employee_id，last_name和其job_id
SELECT employee_id,last_name,job_id
FROM employees e
WHERE 2 <= (
	SELECT COUNT(*)
	FROM job_history j
	WHERE e.employee_id  = j.employee_id 
);

2.3.2、EXISTS 与 NOT EXISTS关键字

  • 关联子查询通常也会和 EXISTS操作符一起来使用，用来检查在子查询中是否存在满足条件的行。
  • 如果在子查询中不存在满足条件的行：
    • 条件返回 FALSE
    • 继续在子查询中查找
  • 如果在子查询中存在满足条件的行：
    • 不在子查询中继续查找
    • 条件返回 TRUE
  • NOT EXISTS关键字表示如果不存在某种条件，则返回TRUE，否则返回FALSE。
-- 查询公司管理者的employee_id，last_name，job_id，department_id信息
-- 方式1：自连接
SELECT DISTINCT mgr.employee_id,mgr.last_name,mgr.job_id,mgr.department_id
FROM employees emp
JOIN employees mgr ON emp.manager_id = mgr.employee_id;
-- 方式2：子查询
SELECT employee_id,last_name,job_id,department_id
FROM employees 
WHERE employee_id IN (
	SELECT DISTINCT manager_id
	FROM employees
);
-- 方式3：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
);

-- 查询departments表中，不存在与employees表中的部门的department_id和department_name
-- 方式1
SELECT d.department_id,department_name
FROM employees e
RIGHT JOIN departments d ON d.department_id = e.department_id
WHERE e.department_id IS NULL;
-- 方式2
SELECT department_id,department_name
FROM departments d
WHERE NOT EXISTS (
	SELECT * 
	FROM employees e
	WHERE d.department_id = e.department_id
);

2.4、子查询的编写技巧

  • 如果子查询相对较简单，建议从外往内写。一旦子查询结构较复杂，则建议从里往外写
  • 如果是相关子查询的话，通常都是从外往里写

三、练习题

-- 1.查询和Zlotkey相同部门的员工姓名和工资 
SELECT last_name,salary
FROM employees 
WHERE department_id IN (
	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。 
SELECT employee_id,last_name,salary
FROM employees
WHERE salary > (
	SELECT AVG(salary) 
	FROM employees
);

-- 3.选择工资大于所有JOB_ID = 'SA_MAN'的员工的工资的员工的last_name, job_id, salary 
-- 方式1
SELECT last_name,job_id,salary
FROM employees
WHERE salary > ALL (
	SELECT salary
	FROM employees
	WHERE job_id = 'SA_MAN'
);
-- 方式2
SELECT last_name,job_id,salary
FROM employees
WHERE salary >  (
	SELECT MAX(salary)
	FROM employees
	WHERE job_id = 'SA_MAN'
);
-- 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 
SELECT employee_id,last_name
FROM employees
WHERE department_id  IN (
	SELECT DISTINCT department_id
	FROM employees 
	WHERE last_name LIKE '%u%'
);

-- 5.查询在部门的location_id为1700的部门工作的员工的员工号 
SELECT employee_id
FROM employees
WHERE department_id IN (
	SELECT department_id
	FROM departments
	WHERE location_id = 1700
);

-- 6.查询管理者是King的员工姓名和工资（谁的管理者是King）
SELECT last_name,salary
FROM employees
WHERE manager_id IN (
	SELECT employee_id
	FROM employees 
	WHERE last_name = 'King'
);

-- 7.查询工资最低的员工信息: last_name, salary 
-- 方式1
SELECT last_name,salary
FROM employees
ORDER BY salary ASC
LIMIT 1;
-- 方式2
SELECT last_name,salary
FROM employees
WHERE salary = (
	SELECT MIN(salary)
	FROM employees
);

-- 8.查询平均工资最低的部门信息 
-- 方式一
SELECT *
FROM departments
WHERE department_id = (
	SELECT department_id
	FROM employees
	GROUP BY department_id
	ORDER BY AVG(salary) ASC
	LIMIT 1
);
-- 方式二
SELECT *
FROM departments
WHERE department_id = (
	SELECT department_id
	FROM employees
	GROUP BY department_id
	HAVING AVG(salary) = (
		SELECT MIN(avg_sal)
		FROM (
			SELECT AVG(salary) avg_sal
			FROM employees
			GROUP BY department_id
		) t_dept_avg_sal
	)
);
-- 方式3
SELECT *
FROM departments
WHERE department_id = (
	SELECT department_id
	FROM employees
	GROUP BY department_id
	HAVING AVG(salary) <= ALL(
		SELECT AVG(salary)
		FROM employees
		GROUP BY department_id
	)
);
-- 方式4
SELECT d.*
FROM departments d,(
	SELECT department_id,AVG(salary) AS "avg_sal"
	FROM employees
	GROUP BY department_id
	ORDER BY avg_sal ASC
	LIMIT 1
) t_dept_avg_sal
WHERE d.department_id = t_dept_avg_sal.department_id;

-- 9.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） 
-- 方式一
SELECT d.*,(
	SELECT AVG(salary)
	FROM employees
	WHERE department_id = d.department_id
) AS "avg_sal"
FROM departments d
WHERE d.department_id = (
	SELECT department_id
	FROM employees
	GROUP BY department_id
	ORDER BY AVG(salary) ASC
	LIMIT 1
);
-- 方式二
SELECT d.*,(
	SELECT AVG(salary)
	FROM employees
	WHERE department_id = d.department_id
) AS "avg_sal"
FROM departments d
WHERE d.department_id = (
	SELECT department_id
	FROM employees
	GROUP BY department_id
	HAVING AVG(salary) = (
		SELECT MIN(avg_sal)
		FROM (
			SELECT AVG(salary) avg_sal
			FROM employees
			GROUP BY department_id
		) t_dept_avg_sal
	)
);
-- 方式3
SELECT d.*,(
	SELECT AVG(salary)
	FROM employees
	WHERE department_id = d.department_id
) AS "avg_sal"
FROM departments d
WHERE d.department_id = (
	SELECT department_id
	FROM employees
	GROUP BY department_id
	HAVING AVG(salary) <= ALL(
		SELECT AVG(salary)
		FROM employees
		GROUP BY department_id
	)
);
-- 方式4
SELECT d.*,(
	SELECT AVG(salary)
	FROM employees
	WHERE department_id = d.department_id
) AS "avg_sal"
FROM departments d,(
	SELECT department_id,AVG(salary) AS "avg_sal"
	FROM employees
	GROUP BY department_id
	ORDER BY avg_sal ASC
	LIMIT 1
) t_dept_avg_sal
WHERE d.department_id = t_dept_avg_sal.department_id;

-- 10.查询平均工资最高的 job 信息 
-- 方式一
SELECT * 
FROM jobs
WHERE job_id = (
	SELECT job_id
	FROM employees 
	GROUP BY job_id
	HAVING AVG(salary) = (
		SELECT MAX(avg_sal)
		FROM (
			SELECT AVG(salary) AS "avg_sal"
			FROM employees 
			GROUP BY job_id
		) t_job_avg_sal
	)
);
-- 方式二
SELECT * 
FROM jobs
WHERE job_id = (
	SELECT job_id
	FROM employees 
	GROUP BY job_id
	HAVING AVG(salary) >= ALL (
		SELECT AVG(salary) 
		FROM employees 
		GROUP BY job_id
	)
);
-- 方式三
SELECT * 
FROM jobs
WHERE job_id = (
	SELECT job_id
	FROM employees 
	GROUP BY job_id
	HAVING AVG(salary) =  (
		SELECT AVG(salary) AS "avg_sal"
		FROM employees 
		GROUP BY job_id
		ORDER BY "avg_sal" DESC
		LIMIT 1
	)
);
-- 方式四
SELECT j.*
FROM jobs j,(
	SELECT job_id,AVG(salary) AS "avg_sal"
	FROM employees 
	GROUP BY job_id
	ORDER BY "avg_sal" DESC
	LIMIT 1
) t_job_avg_sal
WHERE j.job_id = t_job_avg_sal.job_id;

-- 11.查询平均工资高于公司平均工资的部门有哪些? 
SELECT department_id,department_name
FROM departments d
WHERE d.department_id IN (
	SELECT department_id
	FROM employees
	WHERE department_id IS NOT NULL
	GROUP BY department_id
	HAVING AVG(salary) > (
		SELECT AVG(salary)
		FROM employees
	)
);

-- 12.查询出公司中所有 manager 的详细信息 
-- 方式1：自连接
SELECT DISTINCT mgr.employee_id,mgr.last_name,mgr.job_id,mgr.department_id
FROM employees emp
JOIN employees mgr ON emp.manager_id = mgr.employee_id;
-- 方式2：子查询
SELECT employee_id,last_name,job_id,department_id
FROM employees 
WHERE employee_id IN (
	SELECT DISTINCT manager_id
	FROM employees
);
-- 方式3：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
);

-- 13.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? 
-- 方式一
SELECT MIN(salary)
FROM employees
WHERE department_id = (
	SELECT department_id
	FROM employees
	GROUP BY department_id
	HAVING MAX(salary) = (
		SELECT MIN(max_sal)
		FROM (
			SELECT MAX(salary) AS max_sal
			FROM employees
			GROUP BY department_id
		) t_dept_max_sal
	)
);
-- 方式二
SELECT MIN(salary)
FROM employees
WHERE department_id = (
	SELECT department_id
	FROM employees
	GROUP BY department_id
	HAVING MAX(salary) <= ALL(
		SELECT MAX(salary) AS max_sal
		FROM employees
		GROUP BY department_id
	)
);
-- 方式三
SELECT MIN(salary)
FROM employees
WHERE department_id = (
	SELECT department_id
	FROM employees
	GROUP BY department_id
	HAVING MAX(salary) = (
		SELECT MAX(salary) AS max_sal
		FROM employees
		GROUP BY department_id
		ORDER BY max_sal ASC
		LIMIT 1
	)
);
-- 方式四
SELECT MIN(salary)
FROM employees e,(
	SELECT department_id,MAX(salary) AS max_sal
	FROM employees
		GROUP BY department_id
		ORDER BY max_sal ASC
		LIMIT 1
) t_dept_max_sal
WHERE e.department_id = t_dept_max_sal.department_id;

-- 14.查询平均工资最高的部门的 manager 的详细信息: last_name, department_id, email, salary 
-- 方式一
SELECT last_name,department_id,email,salary
FROM employees
WHERE employee_id = ANY (
	SELECT DISTINCT manager_id
	FROM employees
	WHERE department_id = (
		SELECT department_id
		FROM employees
		GROUP BY department_id
		HAVING AVG(salary) = (
			SELECT MAX(avg_sal)
			FROM (
				SELECT AVG(salary) avg_sal
				FROM employees
				GROUP BY department_id
			) t_dept_avg_sal
		)
	)
);
-- 方式二
SELECT last_name,department_id,email,salary
FROM employees
WHERE employee_id = ANY (
	SELECT DISTINCT manager_id
	FROM employees
	WHERE department_id = (
		SELECT department_id
		FROM employees
		GROUP BY department_id
		HAVING AVG(salary) >= ALL (
			SELECT AVG(salary) avg_sal
			FROM employees
			GROUP BY department_id
		)
	)
);
-- 方式三
SELECT last_name,department_id,email,salary
FROM employees 
WHERE employee_id IN (
SELECT DISTINCT manager_id
	FROM employees e,(
		SELECT department_id,AVG(salary) AS "avg_sal"
		FROM employees
		GROUP BY department_id
		ORDER BY avg_sal DESC
		LIMIT 1
	) t_dept_avg_sal
	WHERE e.department_id = t_dept_avg_sal.department_id

);

-- 15. 查询部门的部门号，其中不包括job_id是"ST_CLERK"的部门号 
-- 方式一
SELECT department_id
FROM departments
WHERE department_id NOT IN (
	SELECT DISTINCT department_id
	FROM employees
	WHERE job_id = 'ST_CLERK'
);
-- 方式二
SELECT department_id
FROM departments d
WHERE NOT EXISTS (
	SELECT * 
	FROM employees e
	WHERE d.department_id = e.department_id
	AND e.job_id = 'ST_CLERK'
);

-- 16. 选择所有没有管理者的员工的last_name 
-- 方式一
SELECT last_name
FROM employees emp
WHERE NOT EXISTS (
	SELECT * 
	FROM employees mgr
	WHERE emp.manager_id = mgr.employee_id
);
-- 方式二
SELECT last_name
FROM employees
WHERE manager_id IS NULL;

-- 17．查询员工号、姓名、雇用时间、工资，其中员工的管理者为 'De Haan' 
-- 方式一
SELECT employee_id,last_name,hire_date,salary
FROM employees
WHERE manager_id IN (
	SELECT employee_id
	FROM employees
	WHERE last_name = 'De Haan'
);
-- 方式二
SELECT employee_id,last_name,hire_date,salary
FROM employees emp
WHERE EXISTS (
	SELECT *
	FROM employees mgr
	WHERE emp.manager_id = mgr.employee_id
	AND mgr.last_name = 'De Haan'
);

-- 18.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（相关子查询） 
-- 方式1：相关子查询
SELECT last_name,salary,department_id
FROM employees e1
WHERE salary > (
	SELECT AVG(salary)
	FROM employees e2
	WHERE department_id = e1.department_id 
);
-- 方式2：FROM中声明子查询
SELECT e.last_name,e.salary,e.department_id
FROM employees e,(
	SELECT department_id,AVG(salary) AS "avg_sal"
	FROM employees
	GROUP BY department_id
	) AS t_dept_avg_sal
WHERE e.department_id = t_dept_avg_sal.department_id
AND e.salary > t_dept_avg_sal.avg_sal;

-- 19.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
	WHERE d.department_id = e.department_id
);

-- 20.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 
);
02.SQL与JDBCSQL非关联子查询关联子查询不相关子查询相关子查询多行子查询单行子查询子查询

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