MySQL子查询

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1. 由一个具体的需求引入子查询

需求谁的工资比Abel的高

方式1

SELECT salary
FROM employees
WHERE last_name = 'Abel';
SELECT last_name,salary
FROM employees
WHERE salary > 11000;

方式2自连接

SELECT e2.last_name,e2.salary
FROM employees e1,employees e2
WHERE e2.`salary` > e1.`salary` #多表的连接条件
AND e1.last_name = 'Abel';

方式3子查询

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

2. 称谓的规范外查询或主查询、内查询或子查询

/*
- 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
- 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
*/ 
#不推荐
SELECT last_name,salary
FROM employees
WHERE  (
SELECT salary
FROM employees
WHERE last_name = 'Abel'
) < salary;
/*

3. 子查询的分类

角度1从内查询返回的结果的条目数

单行子查询 vs 多行子查询

角度2内查询是否被执行多次

相关子查询 vs 不相关子查询

比如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。

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。

*/

#子查询的编写技巧或步骤① 从里往外写 ② 从外往里写

4. 单行子查询

4.1 单行操作符 = != > >= < <=

题目查询工资大于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
);

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,job_id和salary

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

题目查询与141号员工的manager_id和department_id相同的其他员工的employee_idmanager_iddepartment_id。

#方式1
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;
#方式2了解
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;

题目查询最低工资大于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
     );

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 "location"
FROM employees;

4.2 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');

4.3 非法使用子查询

#错误Subquery returns more than 1 row
SELECT employee_id, last_name
FROM   employees
WHERE  salary =
                (SELECT   MIN(salary)
                 FROM     employees
                 GROUP BY department_id);

5.多行子查询

5.1 多行子查询的操作符

IN ANY ALL SOME(同ANY)

5.2举例

# IN:
SELECT employee_id, last_name
FROM   employees
WHERE  salary IN
                (SELECT   MIN(salary)
                 FROM     employees
                 GROUP BY department_id); 
                 
# ANY / ALL:

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 job_id <> 'IT_PROG'
AND salary < ANY (
SELECT salary
FROM employees
WHERE 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 job_id <> 'IT_PROG'
AND salary < ALL (
SELECT salary
FROM employees
WHERE job_id = 'IT_PROG'
);

题目查询平均工资最低的部门id

#MySQL中聚合函数是不能嵌套使用的。
#方式1
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2
SELECT department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary) <= ALL(
SELECT AVG(salary) avg_sal
FROM employees
GROUP BY department_id
)

#5.3 空值问题

SELECT last_name

FROM employees

WHERE employee_id NOT IN (

SELECT manager_id

FROM employees

);

6. 相关子查询

#回顾查询员工中工资大于公司平均工资的员工的last_name,salary和其department_id

6.1 

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

题目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使用相关子查询
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在FROM中声明子查询
SELECT e.last_name,e.salary,e.department_id
FROM employees e,(
SELECT department_id,AVG(salary) avg_sal
FROM employees
GROUP BY department_id) t_dept_avg_sal
WHERE e.department_id = t_dept_avg_sal.department_id
AND e.salary > t_dept_avg_sal.avg_sal

题目查询员工的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;
#结论在SELECT中除了GROUP BY 和 LIMIT之外其他位置都可以声明子查询
/*
SELECT ....,....,....(存在聚合函数)
FROM ... (LEFT / RIGHT)JOIN ....ON 多表的连接条件 
(LEFT / RIGHT)JOIN ... ON ....
WHERE 不包含聚合函数的过滤条件
GROUP BY ...,....
HAVING 包含聚合函数的过滤条件
ORDER BY ....,...(ASC / DESC )
LIMIT ...,....
*/

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

SELECT *
FROM job_history;
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`
)

6.2 EXISTS 与 NOT EXISTS关键字

题目查询公司管理者的employee_idlast_namejob_iddepartment_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`
     );

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1
SELECT d.department_id,d.department_name
FROM employees e RIGHT JOIN departments d
ON e.`department_id` = d.`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`
);
SELECT COUNT(*)
FROM departments;

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
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

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

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

SELECT last_name,salary,manager_id
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1
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2
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3 LIMIT
SELECT *
FROM departments
WHERE department_id = (
SELECT department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary ) =(
SELECT AVG(salary) avg_sal
FROM employees
GROUP BY department_id
ORDER BY avg_sal ASC
LIMIT 1
)
);
#方式4
SELECT d.*
FROM departments d,(
SELECT department_id,AVG(salary) avg_sal
FROM employees
GROUP BY department_id
ORDER BY avg_sal ASC
LIMIT 0,1
) 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1
SELECT d.*,(SELECT AVG(salary) FROM employees WHERE department_id = d.`department_id`) avg_sal
FROM departments d
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2
SELECT d.*,(SELECT AVG(salary) FROM employees WHERE department_id = d.`department_id`) avg_sal
FROM departments d
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3 LIMIT
SELECT d.*,(SELECT AVG(salary) FROM employees WHERE department_id = d.`department_id`) avg_sal
FROM departments d
WHERE department_id = (
SELECT department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary ) =(
SELECT AVG(salary) avg_sal
FROM employees
GROUP BY department_id
ORDER BY avg_sal ASC
LIMIT 1
)
);
#方式4
SELECT d.*,(SELECT AVG(salary) FROM employees WHERE department_id = d.`department_id`) avg_sal
FROM departments d,(
SELECT department_id,AVG(salary) avg_sal
FROM employees
GROUP BY department_id
ORDER BY avg_sal ASC
LIMIT 0,1
) 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1
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) avg_sal
FROM employees
GROUP BY job_id
) t_job_avg_sal
)
);
#方式2
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3
SELECT *
FROM jobs
WHERE job_id = (
SELECT job_id
FROM employees
GROUP BY job_id
HAVING AVG(salary) =(
     SELECT AVG(salary) avg_sal
     FROM employees
     GROUP BY job_id
     ORDER BY avg_sal DESC
     LIMIT 0,1
     )
);
#方式4
SELECT j.*
FROM jobs j,(
SELECT job_id,AVG(salary) avg_sal
FROM employees
GROUP BY job_id
ORDER BY avg_sal DESC
LIMIT 0,1
) 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
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  xxx worked for yyy
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`
     );

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1
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) max_sal
FROM employees
GROUP BY department_id
) t_dept_max_sal
)
);
SELECT *
FROM employees
WHERE department_id = 10;
#方式2
SELECT MIN(salary)
FROM employees
WHERE department_id = (
SELECT department_id
FROM employees
GROUP BY department_id
HAVING MAX(salary) <= ALL (
SELECT MAX(salary)
FROM employees
GROUP BY department_id
)
);
#方式3
SELECT MIN(salary)
FROM employees
WHERE department_id = (
SELECT department_id
FROM employees
GROUP BY department_id
HAVING MAX(salary) = (
SELECT MAX(salary) max_sal
FROM employees
GROUP BY department_id
ORDER BY max_sal ASC
LIMIT 0,1
)
);
#方式4
SELECT MIN(salary)
FROM employees e,(
SELECT department_id,MAX(salary) max_sal
FROM employees
GROUP BY department_id
ORDER BY max_sal ASC
LIMIT 0,1
) 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1
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2
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3
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) avg_sal
FROM employees
GROUP BY department_id
ORDER BY avg_sal DESC
LIMIT 0,1
) 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1
SELECT department_id
FROM departments
WHERE department_id NOT IN (
SELECT DISTINCT department_id
FROM employees
WHERE job_id = 'ST_CLERK'
);
#方式2
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

SELECT last_name
FROM employees emp
WHERE NOT EXISTS (
SELECT *
FROM employees mgr
WHERE emp.`manager_id` = mgr.`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'

#方式1
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2
SELECT employee_id,last_name,hire_date,salary
FROM employees e1
WHERE EXISTS (
SELECT *
FROM employees e2
WHERE e1.`manager_id` = e2.`employee_id`
AND e2.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使用相关子查询
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在FROM中声明子查询
SELECT e.last_name,e.salary,e.department_id
FROM employees e,(
SELECT department_id,AVG(salary) avg_sal
FROM employees
GROUP BY department_id) 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 * FROM locations;
SELECT country_id
FROM locations l
WHERE 2 < (
   SELECT COUNT(*)
   FROM departments d
   WHERE l.`location_id` = d.`location_id`
 );

子查询的编写技巧或步骤

① 从里往外写 ② 从外往里写

如何选择

① 如果子查询相对较简单建议从外往里写。一旦子查询结构较复杂则建议从里往外写

② 如果是相关子查询的话通常都是从外往里写。

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