L
“`sql

#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
#工资大于姓Abel的
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

#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

);

“`

最后修改日期:2020年10月15日

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