MySQL学习笔记 ------ 子查询

#进阶7:子查询

/*

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

*/

#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-----即子查询结果必须是一行一列

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

#行子查询

SELECT *

FROM employees

WHERE (employee_id,salary)=(

SELECT MIN(employee_id),MAX(salary)

FROM employees

);

#普通解法

#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-----即子查询结果只能是一行一列

*/

#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

*/

#主要用于子语句的结果是否存在

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`

);

#---------子查询相关案例-----------#

#1.查询和Zlotkey相同部门的员工姓名和工资

#(1)查询和Zlotkey相同部门id

SELECT department_id

FROM employees

WHERE last_name = 'Zlotkey';

#(2)查询员工姓名和工资

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)查询公司平均工资

SELECT AVG(salary)

FROM employees;

#(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.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)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

SELECT e.`employee_id`,e.`last_name`,e.`salary`

FROM `employees` e

JOIN (

SELECT AVG(salary) ag,`department_id`

FROM `employees`

GROUP BY `department_id`

) n

ON e.`department_id`=n.`department_id`

WHERE e.`salary`>n.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id

SELECT DISTINCT `department_id`

FROM `employees`

WHERE last_name LIKE '%u%';

#(2)查询和(1)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

#(1)查询location_id为1700的部门id

SELECT `department_id`

FROM `departments`

WHERE `location_id`=1700;

#(2)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K_ing的员工姓名和工资

#(1)查询K_ing的id

SELECT `employee_id`

FROM `employees`

WHERE `last_name`='K_ing';

#(2)查询管理者是K_ing的员工姓名和工资

SELECT m.`last_name`,m.`salary`

FROM `employees` m

WHERE m.`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

);

#------子查询总结------#

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

二、分类

1、按出现位置

SELECT后面:

仅仅支持标量子查询

FROM后面:

表子查询

WHERE或HAVING后面:

标量子查询

列子查询

行子查询

EXISTS后面:

标量子查询

列子查询

行子查询

表子查询

2、按结果集的行列

标量子查询(单行子查询):结果集为一行一列

列子查询(多行子查询):结果集为多行一列

行子查询:结果集为多行多列

表子查询:结果集为多行多列

三、示例

WHERE或HAVING后面

1、标量子查询

案例:查询最低工资的员工姓名和工资

①最低工资

SELECT MIN(salary) FROM employees

②查询员工的姓名和工资,要求工资=①

SELECT last_name,salary

FROM employees

WHERE salary=(

SELECT MIN(salary) FROM employees

);

2、列子查询

案例:查询所有是领导的员工姓名

①查询所有员工的 manager_id

SELECT manager_id

FROM employees

②查询姓名,employee_id属于①列表的一个

SELECT last_name

FROM employees

WHERE employee_id IN(

SELECT manager_id

FROM employees

);

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