含义
出现在其他语句中的select语句,称为子查询或内查询。
例如下面的代码就属于子查询:
SELECT first_name from employees where
department_id in(
select department_id from departments
where location_id=1700
)
在条件当中又插入一个条件,插入的条件可以单独执行。
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(例如:> < >= <= = <>)
④列子查询,一般搭配着多行操作符使用(例如:in、any/some、all)
⑤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
标量子查询(单行子查询)
案例1:谁的工资比 Abel 高?
应该首先查询Abel的工资,然后查询比她工资高的人
①查询Abel的工资
SELECT salary
FROM employees
WHERE last_name = 'Abel'
查询的结果只有一行一列,所以称为标量子查询 。
②查询员工的名字,满足 salary>①结果
SELECT first_name
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查询最低工资大于50号部门的
SELECT MIN(salary),department_id
FROM employees
GROUP BY department_id
HAVING MIN(salary)>(
SELECT MIN(salary)
FROM employees
WHERE department_id = 50
);
列子查询(多行子查询)
- 返回多行
- 使用多行比较操作符
操作符 | 含义 |
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进行比较 |
ALL | 和子查询返回的所有值比较 |
a in(10,20,30)就是判断a是否为10,20,30中的任意一个,a> any(10,20,30)就是a是否都大于10,20,30中的任意一个就可以。a> any(10,20,30)就是a大于10,20,30中所有数。
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 department_id
FROM departments
WHERE location_id IN(1400,1700)
可以看出查询结果是单列多行,所以是列子查询。
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';
行子查询(结果集一行多列或多行多列)
案例1:查询员工编号最小并且工资最高的员工信息
①查询最小的员工编号
SELECT MIN(employee_id)
FROM employees
②查询最高工资的员工编号
SELECT employee_id,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
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
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;
练习题
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查询和①部门的员工姓名和工资
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①高的员工的员工号,姓名和工资
SELECT job_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 department_id,AVG(salary)
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 ;