本文目录
一.子查询概述:
1.含义:
出现在其他语句中的select语句,称为子查询或内查询,而外部的查询语句,称为主查询或外查询。
2.分类:
1).(本文主要讲from和where、having后面的子查询)
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
2).
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or多行多列)
表子查询(结果集一般为多行多列)
3.注意事项:
1)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
2)子查询放在小括号内
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。
二.放在where或者having后面
1.标量子查询
1.1子查询放在where后
#案例1:谁的工资比 Abel 高?
#step1:查询Abel的工资
SELECT salary
FROM employees
WHERE last_name = 'Abel';
#step2:查询员工的信息,满足 salary>step1结果
SELECT last_name,salary
FROM employees
WHERE salary>(
SELECT salary
FROM employees
WHERE last_name = 'Abel'
);
step1的结果是:
可以看到,查询的结果只有一行一列,所以叫标量子查询。
step2的结果是:
#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:
#step1:查询141号员工的job_id
SELECT job_id
FROM employees
WHERE employee_id = 141;
#step2:查询143号员工的salary
SELECT salary
FROM employees
WHERE employee_id = 143;
#step3:查询员工的姓名,job_id 和工资,要求job_id=step1结果并且salary>step2结果
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);
step1的结果:
step2的结果:
step3:
1.2子查询放在having后
#案例3: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
#step1:查询50号部门的最低工资
select MIN(salary) as 50号部门的最低工资
from employees
where department_id = 50 ;
#step2:查询其他部门的最低工资
select department_id,MIN(salary) as 该部门最低工资
from employees
group by department_id
#step3:在step2查询的结果基础上,添加having条件
select department_id,MIN(salary) as 该部门最低工资
from employees
group by department_id
having 该部门最低工资>(
SELECT MIN(salary) as 50号部门的最低工资
FROM employees
WHERE department_id = 50);
step3的结果:
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
#step1:查询location_id是1400和1700的部门的部门编号
SELECT DISTINCT department_id AS 部门编号
FROM departments
WHERE location_id IN (1400,1700);
#step2:查询员工姓名,要求员工的部门是step1结果中的一个
SELECT last_name AS 员工姓名, department_id AS 所属部门
FROM employees
WHERE department_id IN (
SELECT DISTINCT department_id AS 部门编号
FROM departments
WHERE location_id IN (1400,1700)
);
step1的结果:
step2的结果:
3.行子查询
#案例:查询job_id 和salary都和Austin一样的员工姓名、job_id和salary
#step1:查询Austin的job_id和salary
SELECT job_id AS Austin的工种,salary AS Austin的工资
FROM employees
WHERE last_name = 'austin';
#step1的查询结果是一行,所以叫行子查询
#step2:
SELECT last_name,job_id,salary
FROM employees
WHERE (job_id,salary)=(
SELECT job_id AS Austin的工种,salary AS Austin的工资
FROM employees
WHERE last_name = 'austin')
AND last_name <> 'austin';
step1的结果:
step2的结果:
三.放到from后面
PS:子查询结果充当一张表,要求必须起别名
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#案例:查询每个部们平均工资的工资等级
#step1:查询每个部们的平均工资
SELECT department_id AS 部门编号,AVG(salary) AS 该部门平均工资
FROM employees
GROUP BY department_id;
#step2:
SELECT de_sal.部门编号, sal_g.`grade_level` AS 该部门平均工资等级
FROM (SELECT department_id AS 部门编号,AVG(salary) AS 该部门平均工资
FROM employees
GROUP BY department_id) AS de_sal
INNER JOIN salary_grades sal_g
ON de_sal.该部门平均工资 BETWEEN sal_g.`lowest_sal` AND sal_g.`highest_sal`;
大概就这么多内容吧,,,如果后面还想到了其他的再添加…