MySQL的DQL子查询 - 码农教程

MySQL列表页

含义：

出现在其他语句中的select语句，称为子查询或内查询

外部的查询语句，称为主查询或外查询

分类：

按子查询出现的位置：

select后面：

仅仅支持标量子查询

from后面：

支持表子查询

where或having后面：★

标量子查询（单行） √

列子查询（多行） √

行子查询

exists后面（相关子查询）

表子查询

按结果集的行列数不同：

标量子查询（结果集只有一行一列）

列子查询（结果集只有一列多行）

行子查询（结果集有一行多列）

表子查询（结果集一般为多行多列）

一、where或having后面

1、标量子查询（单行子查询）

2、列子查询（多行子查询）

3、行子查询（多列多行）

特点：

①子查询放在小括号内

②子查询一般放在条件的右侧

③标量子查询，一般搭配着单行操作符使用

> < >= <= = <>

列子查询，一般搭配着多行操作符使用

in、any/some、all

④子查询的执行优先于主查询执行，主查询的条件用到了子查询的结果

多行比较操作符

IN/NOT IN 等于列表中的任意一个

ANY|SOME 和子查询返回的某一个值比较

ALL 和子查询返回的所有值比较

1》标量子查询★

案例1：谁的工资比 Abel 高?

①查询Abel的工资

SELECT salary
FROM employees
WHERE last_name = 'Abel'

②查询员工的信息，满足 salary>①结果

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

案例2：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

①查询141号员工的job_id

SELECT job_id
FROM employees
WHERE employee_id = 141

②查询143号员工的salary

SELECT salary
FROM employees
WHERE employee_id = 143

③查询员工的姓名，job_id 和工资，要求job_id=①并且salary>②

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
);

#案例3：返回公司工资最少的员工的last_name,job_id和salary

①查询公司的最低工资

SELECT MIN(salary)
FROM employees

②查询last_name,job_id和salary，要求salary=①

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

案例4：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

①查询50号部门的最低工资

SELECT  MIN(salary)
FROM employees
WHERE department_id = 50

②查询每个部门的最低工资

SELECT MIN(salary),department_id
FROM employees
GROUP BY department_id

③ 在②基础上筛选，满足min(salary)>①

SELECT MIN(salary),department_id
FROM employees
GROUP BY department_id
HAVING MIN(salary)>(
    SELECT  MIN(salary)
    FROM employees
    WHERE department_id = 50
);

非法使用标量子查询

SELECT MIN(salary),department_id
FROM employees
GROUP BY department_id
HAVING MIN(salary)>(
    SELECT  salary
    FROM employees
    WHERE department_id = 250
);

2》列子查询（多行子查询）★

案例1：返回location_id是1400或1700的部门中的所有员工姓名

①查询location_id是1400或1700的部门编号

SELECT DISTINCT department_id
FROM departments
WHERE location_id IN(1400,1700)

②查询员工姓名，要求部门号是①列表中的某一个

SELECT last_name
FROM employees
WHERE department_id  <>ALL(
    SELECT DISTINCT department_id
    FROM departments
    WHERE location_id IN(1400,1700)
);

案例2：返回其它工种中比job_id为‘IT_PROG’工种任一工资低的员工的员工号、姓名、job_id 以及salary

①查询job_id为‘IT_PROG’部门任一工资

SELECT DISTINCT salary
FROM employees
WHERE job_id = 'IT_PROG'

②查询员工号、姓名、job_id 以及salary，salary<(①)的任意一个

SELECT last_name,employee_id,job_id,salary
FROM employees
WHERE salary<ANY(
    SELECT DISTINCT salary
    FROM employees
    WHERE job_id = 'IT_PROG'
) AND job_id<>'IT_PROG';

或

SELECT last_name,employee_id,job_id,salary
FROM employees
WHERE salary<(
    SELECT MAX(salary)
    FROM employees
    WHERE job_id = 'IT_PROG'
) AND job_id<>'IT_PROG';

案例3：返回其它部门中比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 last_name,employee_id,job_id,salary
FROM employees
WHERE salary<ALL(
    SELECT DISTINCT salary
    FROM employees
    WHERE job_id = 'IT_PROG'
) AND job_id<>'IT_PROG';

或

SELECT last_name,employee_id,job_id,salary
FROM employees
WHERE salary<(
    SELECT MIN( salary)
    FROM employees
    WHERE job_id = 'IT_PROG'
) AND job_id<>'IT_PROG';

3》行子查询（结果集一行多列或多行多列）

案例：查询员工编号最小并且工资最高的员工信息

SELECT *
FROM employees
WHERE (employee_id,salary)=(
    SELECT MIN(employee_id),MAX(salary)
    FROM employees
);

①查询最小的员工编号

SELECT MIN(employee_id)
FROM employees

②查询最高工资

SELECT MAX(salary)
FROM employees

③查询员工信息

SELECT *
FROM employees
WHERE employee_id=(
    SELECT MIN(employee_id)
    FROM employees
)AND salary=(
    SELECT MAX(salary)
    FROM employees
);

二、select后面

仅仅支持标量子查询

案例：查询每个部门的员工个数

SELECT d.*,(
    SELECT COUNT(*)
    FROM employees e
    WHERE e.department_id = d.`department_id`
 ) 个数
 FROM departments d;

案例2：查询员工号=102的部门名

SELECT (
    SELECT department_name,e.department_id
    FROM departments d
    INNER JOIN employees e
    ON d.department_id=e.department_id
    WHERE e.employee_id=102
) 部门名;

三、from后面

将子查询结果充当一张表，要求必须起别名

案例：查询每个部门的平均工资的工资等级

①查询每个部门的平均工资

SELECT AVG(salary),department_id
FROM employees
GROUP BY department_id

SELECT * FROM job_grades;

②连接①的结果集和job_grades表，筛选条件平均工资 between lowest_sal and highest_sal

SELECT  ag_dep.*,g.`grade_level`
FROM (
    SELECT AVG(salary) ag,department_id
    FROM employees
    GROUP BY department_id
) ag_dep
INNER JOIN job_grades g
ON ag_dep.ag BETWEEN lowest_sal AND highest_sal;

四、exists后面（相关子查询）

语法：

exists(完整的查询语句)

结果：

1或0

SELECT EXISTS(SELECT employee_id FROM employees WHERE salary=300000);

案例1：查询有员工的部门名

SELECT department_name
FROM departments d
WHERE d.`department_id` IN(
    SELECT department_id
    FROM employees
)

exists

SELECT department_name
FROM departments d
WHERE EXISTS(
    SELECT *
    FROM employees e
    WHERE d.`department_id`=e.`department_id`
);

案例2：查询没有女朋友的男神信息

SELECT bo.*
FROM boys bo
WHERE bo.id NOT IN(
    SELECT boyfriend_id
    FROM beauty
)

exists

SELECT bo.*
FROM boys bo
WHERE NOT EXISTS(
    SELECT boyfriend_id
    FROM beauty b
    WHERE bo.`id`=b.`boyfriend_id`
);

五、子查询例题

1. 查询和Zlotkey相同部门的员工姓名和工资

①查询Zlotkey的部门

SELECT department_id
FROM employees
WHERE last_name = 'Zlotkey'

②查询部门号=①的姓名和工资

SELECT last_name,salary
FROM employees
WHERE 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。

①查询平均工资

SELECT AVG(salary)
FROM employees

②查询工资>①的员工号，姓名和工资。

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

3.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

①查询各部门的平均工资

SELECT AVG(salary),department_id
FROM employees
GROUP BY department_id

②连接①结果集和employees表，进行筛选

SELECT employee_id,last_name,salary,e.department_id
FROM employees e
INNER JOIN (
    SELECT AVG(salary) ag,department_id
    FROM employees
    GROUP BY department_id
) ag_dep
ON e.department_id = ag_dep.department_id
WHERE salary>ag_dep.ag ;

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

①查询姓名中包含字母u的员工的部门

SELECT  DISTINCT department_id
FROM employees
WHERE last_name LIKE '%u%'

②查询部门号=①中的任意一个的员工号和姓名

SELECT last_name,employee_id
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

①查询location_id为1700的部门

SELECT DISTINCT department_id
FROM departments
WHERE location_id  = 1700

②查询部门号=①中的任意一个的员工号

SELECT employee_id
FROM employees
WHERE department_id =ANY(
    SELECT DISTINCT department_id
    FROM departments
    WHERE location_id  = 1700
);

6.查询管理者是King的员工姓名和工资

①查询姓名为king的员工编号

SELECT employee_id
FROM employees
WHERE last_name  = 'K_ing'

②查询哪个员工的manager_id = ①

SELECT last_name,salary
FROM employees
WHERE manager_id IN(
    SELECT employee_id
    FROM employees
    WHERE last_name  = 'K_ing'
);

7.查询工资最高的员工的姓名，要求first_name和last_name显示为一列，列名为姓.名

①查询最高工资

SELECT MAX(salary)
FROM employees

②查询工资=①的姓.名

SELECT CONCAT(first_name,last_name) "姓.名"
FROM employees
WHERE salary=(
    SELECT MAX(salary)
    FROM employees
);

六、子查询经典案例

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 employee
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) ;