DQL 语言 --子查询 10

一、含义

　　嵌套在其他语句内部的select语句称为子查询或内查询，
　　外面的语句可以是insert、update、delete、select等，一般select作为外面语句较多
　　外面如果为select语句，则此语句称为外查询或主查询

#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）

　　　　　　in：等于列表中任意一个
　　　　　　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

　　　　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

#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>(
    #即使 Abel 信息发生改变，查询结果也会跟着变化 
    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)>(     #-----------------------  > 只能对应一条数据 ，而id = 50 有很多条，则会报错
    SELECT  salary
    FROM employees
    WHERE department_id = 50


);

#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（也可用连接查询）

#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  IN(
    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';

写法二：

#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;
);                    # ----------------合并书写

#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           #-------------分开写
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1：查询每个部门的员工个数

SELECT d.*,(              #  -----如果用分组查询，部门员工个数为0 的 无法统计

    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 (               #-----------------括号里面只能单行单列（标准子查询）
    SELECT department_name
    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 (    #--------------------实质上是一个表
    SELECT AVG(salary) ag,department_id
    FROM employees
    GROUP BY department_id
) ag_dep  #----------------------取别名
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

#in
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

#in
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`

);

测试：

#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

);

posted on 2020-07-31 13:49 lfw123 阅读(151) 评论(0) 编辑收藏举报

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DQL 语言 --子查询 10

一、含义

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