【SQL查询系列】子查询经典案例

前言

近期在补SQL的基础，特别整理了一下SQL的一些查询练习语句，边学边笔记，持续更新~

数据库

查询案例

1. 查询工资最低的员工信息: last_name, salary

#①查询最低的工资
SELECT MIN(salary)
FROM employees
#②查询last_name,salary，要求salary=①
SELECT last_name,salary
FROM employees
WHERE salary=(
    SELECT MIN(salary)
    FROM employees
);

2. 查询平均工资最低的部门信息

#方式一：
#①各部门的平均工资
SELECT AVG(salary),department_id
FROM employees
GROUP BY department_id
#②查询①结果上的最低平均工资
SELECT MIN(ag)
FROM (
    SELECT AVG(salary) ag,department_id
    FROM employees
    GROUP BY department_id
) ag_dep
#③查询哪个部门的平均工资=②
SELECT AVG(salary),department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary)=(
    SELECT MIN(ag)
    FROM (
        SELECT AVG(salary) ag,department_id
        FROM employees
        GROUP BY department_id
    ) ag_dep
);
#④查询部门信息
SELECT d.*
FROM departments d
WHERE d.`department_id`=(
    SELECT department_id
    FROM employees
    GROUP BY department_id
    HAVING AVG(salary)=(
        SELECT MIN(ag)
        FROM (
            SELECT AVG(salary) ag,department_id
            FROM employees
            GROUP BY department_id
        ) ag_dep
    )
);
#方式二：
#①各部门的平均工资
SELECT AVG(salary),department_id
FROM employees
GROUP BY department_id
#②求出最低平均工资的部门编号
SELECT department_id
FROM employees
GROUP BY department_id
ORDER BY AVG(salary) 
LIMIT 1;
#③查询部门信息
SELECT *
FROM departments
WHERE department_id=(
    SELECT department_id
    FROM employees
    GROUP BY department_id
    ORDER BY AVG(salary) 
    LIMIT 1
);

3. 查询平均工资最低的部门信息和该部门的平均工资

#①各部门的平均工资
SELECT AVG(salary),department_id
FROM employees
GROUP BY department_id
#②求出最低平均工资的部门编号
SELECT AVG(salary),department_id
FROM employees
GROUP BY department_id
ORDER BY AVG(salary) 
LIMIT 1;
#③查询部门信息
SELECT d.*,ag
FROM departments d
JOIN (
    SELECT AVG(salary) ag,department_id
    FROM employees
    GROUP BY department_id
    ORDER BY AVG(salary) 
    LIMIT 1
) ag_dep
ON d.`department_id`=ag_dep.department_id;

4. 查询平均工资最高的 job 信息

#①查询最高的job的平均工资
SELECT AVG(salary),job_id
FROM employees
GROUP BY job_id
ORDER BY AVG(salary) DESC
LIMIT 1
#②查询job信息
SELECT * 
FROM jobs
WHERE job_id=(
    SELECT job_id
    FROM employees
    GROUP BY job_id
    ORDER BY AVG(salary) DESC
    LIMIT 1
);

5. 查询平均工资高于公司平均工资的部门有哪些?

#①查询平均工资
SELECT AVG(salary)
FROM employees
#②查询每个部门的平均工资
SELECT AVG(salary),department_id
FROM employees
GROUP BY department_id
#③筛选②结果集，满足平均工资>①
SELECT AVG(salary),department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary)>(
    SELECT AVG(salary)
    FROM employees
);

6. 查询出公司中所有 manager 的详细信息.

#①查询所有manager的员工编号
SELECT DISTINCT manager_id
FROM employees
#②查询详细信息，满足employee_id=①
SELECT *
FROM employees
WHERE employee_id =ANY(
    SELECT DISTINCT manager_id
    FROM employees
);

7. 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

#①查询各部门的最高工资中最低的部门编号
SELECT department_id
FROM employees
GROUP BY department_id
ORDER BY MAX(salary)
LIMIT 1
#②查询①结果的那个部门的最低工资
SELECT MIN(salary) ,department_id
FROM employees
WHERE department_id=(
    SELECT department_id
    FROM employees
    GROUP BY department_id
    ORDER BY MAX(salary)
    LIMIT 1
);

8. 查询平均工资最高的部门的 manager 的详细信息: last_name, department_id, email, salary

#①查询平均工资最高的部门编号
SELECT 
    department_id 
FROM
    employees 
GROUP BY department_id 
ORDER BY AVG(salary) DESC 
LIMIT 1 
#②将employees和departments连接查询，筛选条件是①
    SELECT 
        last_name, d.department_id, email, salary 
    FROM
        employees e 
        INNER JOIN departments d 
            ON d.manager_id = e.employee_id 
    WHERE d.department_id = 
        (SELECT 
            department_id 
        FROM
            employees 
        GROUP BY department_id 
        ORDER BY AVG(salary) DESC 
        LIMIT 1) ;

student数据库

查询练习

一、查询每个专业的学生人数SELECT majorid,COUNT(*)

FROM student
GROUP BY majorid;

二、查询参加考试的学生中，每个学生的平均分、最高分

SELECT AVG(score),MAX(score),studentno
FROM result
GROUP BY studentno;

三、查询姓张的每个学生的最低分大于60的学号、姓名

SELECT s.studentno,s.`studentname`,MIN(score)
FROM student s
JOIN result r
ON s.`studentno`=r.`studentno`
WHERE s.`studentname` LIKE '张%'
GROUP BY s.`studentno`
HAVING MIN(score)>60;

四、查询每个专业生日在“1988-1-1”后的学生姓名、专业名称

SELECT m.`majorname`,s.`studentname`
FROM student s
JOIN major m
ON m.`majorid`=s.`majorid`
WHERE DATEDIFF(borndate,'1988-1-1')>0
GROUP BY m.`majorid`;

五、查询每个专业的男生人数和女生人数分别是多少

SELECT COUNT(*),sex,majorid
FROM student
GROUP BY sex,majorid;

六、查询专业和张翠山一样的学生的最低分

#①查询张翠山的专业编号
SELECT majorid
FROM student
WHERE studentname = '张翠山'
#②查询编号=①的所有学生编号
SELECT studentno
FROM student
WHERE majorid=(
    SELECT majorid
    FROM student
    WHERE studentname = '张翠山'
)
#②查询最低分
SELECT MIN(score)
FROM result
WHERE studentno IN(
    SELECT studentno
    FROM student
    WHERE majorid=(
        SELECT majorid
        FROM student
        WHERE studentname = '张翠山'
    )
)

七、查询大于60分的学生的姓名、密码、专业名

SELECT studentname,loginpwd,majorname
FROM student s
JOIN major m ON s.majorid=  m.majorid
JOIN result r ON s.studentno=r.studentno
WHERE r.score>60;

八、按邮箱位数分组，查询每组的学生个数

SELECT COUNT(*),LENGTH(email)
FROM student
GROUP BY LENGTH(email);

九、查询学生名、专业名、分数

SELECT studentname,score,majorname
FROM student s
JOIN major m ON s.majorid=  m.majorid
LEFT JOIN result r ON s.studentno=r.studentno

十、查询哪个专业没有学生，分别用左连接和右连接实现

#左
SELECT m.`majorid`,m.`majorname`,s.`studentno`
FROM major m
LEFT JOIN student s ON m.`majorid` = s.`majorid`
WHERE s.`studentno` IS NULL;
#右
SELECT m.`majorid`,m.`majorname`,s.`studentno`
FROM student s
RIGHT JOIN  major m ON m.`majorid` = s.`majorid`
WHERE s.`studentno` IS NULL;

十一、查询没有成绩的学生人数

SELECT COUNT(*)
FROM student s
LEFT JOIN result r ON s.`studentno` = r.`studentno`
WHERE r.`id` IS NULL