MySQL---子查询

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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，这个特性从MySQL 4.1开始引入。

SQL 中子查询的使用大大增强了 SELECT 查询的能力，因为很多时候查询需要从结果集中获取数据，或者需要从同一个表中先计算得出一个数据结果，然后与这个数据结果（可能是某个标量，也可能是某个集合）进行比较。

需求分析与问题解决

实际问题

• 谁的工资比Abel高？

为了解决这个问题，我们需要先查出Abel的工资是多少

• 方式一
• 方式二
• 方式三

SELECT salary
FROM employees
WHERE last_name = 'Abel';   -- 查询到Abel的工资是11000

SELECT last_name,salary
FROM employees
WHERE salary > 11000;       -- 然后再用表中所有员工的工资与Abel的工资进行比较

我们可以采用自查询的方式

SELECT b.last_name,b.salary
FROM employees a,employees b
WHERE a.`last_name` = 'Abel'
AND b.`salary` > a.`salary`;

子查询，在方式一的基础上，将11000替换成我们查询Abel工资的语句，并用括号包裹

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

子查询的基本使用

• 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 。

• 注意事项
  • 子查询要包含在括号内
  • 最好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

-- 放在左侧会影响代码可读性
SELECT last_name,salary
FROM employees
WHERE  (
       SELECT salary
    FROM employees
    WHERE last_name = 'Abel'
        ) < salary;

子查询的分类

• 分类方式1：
  • 我们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 。
• 分类方式2：
  • 我们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。
  • 同样，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。

单行子查询

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

代码示例

题目：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
        );

题目：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

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);

题目：返回公司工资最少的员工的last_name

SELECT last_name,job_id,salary
FROM employees
WHERE salary = (
        SELECT MIN(salary)
        FROM employees
        );

题目：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

• 方式一
• 方式二

SELECT employee_id,manager_id,department_id
FROM employees
WHERE manager_id = (
            SELECT manager_id
            FROM employees
            WHERE employee_id = 141
           )
AND department_id = (
            SELECT department_id
            FROM employees
            WHERE employee_id = 141
           )
AND employee_id != 141; 

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;

HAVING 中的子查询

• 首先执行子查询。
• 向主查询中的HAVING 子句返回结果。

题目：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

SELECT department_id,MIN(salary) "min_sal"
FROM employees
WHERE department_id IS NOT NULL
GROUP BY department_id
HAVING min_sal > (
    SELECT MIN(salary)
    FROM employees
    WHERE department_id = 50
    );

CASE 中的子查询

题目：显式员工的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) AS "location"
FROM employees;

子查询中的空值问题

SELECT last_name, job_id
FROM   employees
WHERE  job_id =
                (SELECT job_id
                 FROM   employees
                 WHERE  last_name = 'Haas');

没有叫Haas的人，子查询不返回任何行

非法使用子查询

SELECT employee_id, last_name
FROM employees
WHERE salary =
                (SELECT MIN(salary)
                FROM employees
                GROUP BY department_id);

错误信息：Subquery returns more than 1 row
说明：括号中的子查询语句会返回每个部门的最低薪资，远不止一行

多行子查询

• 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

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

代码示例

题目：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';

题目：返回其它job_id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 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';

题目：查询平均工资最低的部门id

在MySQL中，聚合函数不支持嵌套，但是在Oracle可以嵌套

SELECT department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary) <= ALL (SELECT AVG(salary)
                FROM employees
                GROUP BY department_id);

空值问题

内查询中出现NULL，会影响查询结果

SELECT last_name
FROM employees
WHERE employee_id NOT IN (
            SELECT manager_id
            FROM employees
            );

SELECT last_name
FROM employees
WHERE employee_id NOT IN (
            SELECT manager_id
            FROM employees
            WHERE manager_id IS NOT NULL    -- 加一行判断，排除NULL的情况
            );

相关子查询

相关子查询执行流程

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 。

例如查询比Abel工资高的员工，这就不是一个相关子查询，因为我们只需要查询一次，即查询Abel的工资是多少，然后把它当成一个常量来进行比较

代码示例

题目：查询员工中工资大于本部门平均工资的员工的last_name

先来回顾一下之前学的东西，如果改成查询工资大于本公司平均工资的员工，那么代码如下

SELECT last_name,salary,department_id
FROM employees
WHERE salary > (
        SELECT AVG(salary) 
        FROM employees
        );

这里仍然是不相关子查询，因为内查询只查询了一次，即找到公司平均工资即可

那我们现在再来看看原题该怎么做

• 方式一
• 方式二

SELECT last_name,salary,department_id
FROM employees e1
WHERE salary > (
        SELECT AVG(salary) 
        FROM employees e2
        WHERE e1.`department_id`=e2.`department_id`
        );

在 FROM 中使用子查询
查询各部门id的平均工资，当做一张表，然后再多表查询

SELECT e.last_name,e.salary,e.department_id
FROM employees e
JOIN (SELECT department_id,AVG(salary) avg_sal
      FROM employees
      GROUP BY department_id
      ) "t_dept_avg_sal"
ON e.`department_id` = t_dept_avg_sal.department_id
WHERE e.`salary` > t_dept_avg_sal.avg_sal;

题目：查询员工的id，salary，按照department_name 排序

• 方式一
• 方式二

SELECT employee_id,salary
FROM employees e
ORDER BY (
    SELECT department_name
    FROM departments d
    WHERE e.`department_id` = d.`department_id`
)ASC;    

左外连接也可以做

SELECT employee_id,salary
FROM employees e
LEFT JOIN departments d
ON e.`department_id` = d.`department_id`
ORDER BY d.department_name;

题目：若employees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

-- 每次读题都得读个半天才知道啥意思
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` 
        );

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。

题目：查询公司管理者的employee_id，last_name，job_id，department_id信息

• 方式一
• 方式二
• 方式三

自连接

SELECT DISTINCT mng.employee_id,mng.last_name,mng.job_id,mng.department_id
FROM employees emp
JOIN employees mng
ON emp.`manager_id` = mng.`employee_id`;

子查询

SELECT DISTINCT employee_id,last_name,job_id,department_id
FROM employees e
WHERE e.`employee_id` 
IN (
    SELECT manager_id
    FROM employees
    );    

SELECT employee_id,last_name,job_id,department_id
FROM employees e
WHERE EXISTS (SELECT * 
          FROM employees m 
          WHERE e.`employee_id` = m.`manager_id`
          );

题目：查询departments表中，不存在于employees表中的部门的department_id和department_name

• 方式一
• 方式二

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;

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。

不存在e表中的department_id和d表中的的department_id，则返回TRUE，该条记录也正是我们需要的

SELECT d.department_id,d.department_name
FROM departments d
WHERE NOT EXISTS (SELECT department_id
          FROM employees e
          WHERE e.`department_id` = d.`department_id`
          );

相关更新

题目：在employees中增加一个department_name字段，数据为员工对应的部门名称

UPDATE employees e
SET department_name = (SELECT department_name
                       FROM departments d
                       WHERE e.department_id = d.department_id)

相关删除

删除表employees中，其与emp_history表皆有的数据

DELETE FROM employees e
WHERE employee_id IN
(SELECT employee_id
FROM emp_history
WHERE employee_id = e.employee_id);

思考题

问题：谁的工资比Abel的高？

• 方式一
• 方式二

SELECT e2.last_name,e2.salary
FROM employees e1,employees e2
WHERE e1.last_name = 'Abel'
AND e1.`salary` < e2.`salary`;

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

• 以上两种方式有好坏之分吗？
• 答：自连接方式好！
  题目中可以使用子查询，也可以使用自连接。一般情况建议你使用自连接，因为在许多 DBMS 的处理过程中，对于自连接的处理速度要比子查询快得多。可以这样理解：子查询实际上是通过未知表进行查询后的条件判断，而自连接是通过已知的自身数据表进行条件判断，因此在大部分 DBMS 中都对自连接处理进行了优化

课后练习

1. 查询和Zlotkey相同部门的员工姓名和工资

SELECT last_name,salary
FROM employees e
WHERE e.`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。

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

• 方式一
• 方式二

SELECT last_name,job_id,salary
FROM employees
WHERE salary > (
        SELECT MAX(salary)
        FROM employees
        WHERE job_id = 'SA_MAN'
        );

SELECT last_name,job_id,salary
FROM employees
WHERE salary > ALL (
        SELECT 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

• 方式一
• 方式二

SELECT employee_id,last_name
FROM employees a
WHERE department_id IN (SELECT DISTINCT department_id
            FROM employees b
            WHERE  b.`last_name` LIKE "%u%"
            );

SELECT employee_id,last_name
FROM employees a
WHERE department_id = ANY (SELECT DISTINCT department_id
            FROM employees b
            WHERE  b.`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,last_name
FROM employees e
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

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

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
• 方式二

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
                          ));

SELECT d.*
FROM departments d,(
SELECT department_id,AVG(salary) avg_sal
FROM employees
GROUP BY department_id
ORDER BY avg_sal
LIMIT 0,1) e
WHERE d.`department_id` = e.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）

SELECT *
FROM departments d,(
SELECT department_id,AVG(salary) avg_sal
FROM employees
GROUP BY department_id
ORDER BY avg_sal
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
• 方式二

SELECT j.*
FROM jobs j,(
SELECT AVG(salary) avg_sal,job_id
FROM employees
GROUP BY job_id
ORDER BY avg_sal DESC
LIMIT 1
) t_job_id_avg_sal
WHERE j.`job_id` =  t_job_id_avg_sal.job_id;

SELECT *
FROM jobs j
WHERE j.`job_id` = (
        SELECT job_id
        FROM employees
        GROUP BY job_id
        HAVING AVG(salary) >= ALL(
                                SELECT AVG(salary) avg_sal
                                FROM employees
                                GROUP BY 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
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

• 方式一
• 方式二
• 方式三

SELECT DISTINCT mng.*
FROM employees emp , employees mng
WHERE mng.`employee_id` = emp.`manager_id`;

SELECT * 
FROM employees
WHERE employee_id IN (SELECT DISTINCT manager_id
                      FROM employees        
                      );

SELECT * 
FROM employees m
WHERE EXISTS (SELECT DISTINCT *
              FROM employees e
              WHERE m.`employee_id` = e.`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
• 方式二

SELECT employee_id,MIN(salary)
FROM employees e,(SELECT department_id
                        FROM employees
                        WHERE department_id IS NOT NULL
                        GROUP BY department_id
                        ORDER BY MAX(salary)
                        LIMIT 1
                        ) t_max_sal
WHERE e.`department_id` = t_max_sal.department_id;

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)
                        LIMIT 1
                        );

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
• 方式二

SELECT * 
FROM employees
WHERE employee_id = (SELECT manager_id
                     FROM departments 
                     WHERE department_id = (SELECT department_id
                                            FROM employees
                                            GROUP BY department_id
                                            ORDER BY AVG(salary) DESC
                                            LIMIT 1));

SELECT * 
FROM employees
WHERE employee_id IN (SELECT manager_id 
                        FROM employees
                        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
                                                                        ))

15. 查询部门的部门号，其中不包括job_id是"ST_CLERK"的部门号

SELECT department_id
FROM departments
WHERE department_id 
NOT IN (SELECT DISTINCT department_id
	FROM employees
	WHERE job_id = 'ST_CLERK')

16. 选择所有没有管理者的员工的last_name

• 方式一
• 方式二

SELECT last_name
FROM employees
WHERE manager_id IS NULL;

SELECT last_name
FROM employees e1
WHERE NOT EXISTS (
		SELECT *
		FROM employees e2
		WHERE e1.`manager_id` = e2.`employee_id`);

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’

SELECT employee_id,last_name,hire_date,salary
FROM employees
WHERE manager_id = (SELECT employee_id
		    FROM employees
		    WHERE 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）

SELECT employee_id,last_name,salary
FROM employees e1
WHERE salary > (SELECT AVG(salary)
		FROM employees e2
		WHERE  e1.`department_id` = e2.`department_id`);

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_id,department_name
FROM departments d
WHERE 5 < (SELECT COUNT(*)
	   FROM employees e
	   WHERE e.`department_id` = d.`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 d.`location_id` = l.`location_id`
	);