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(用的较少)
exists后面(相关子查询)
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)
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Able高?
#①查询Able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
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last_name,job_id和salary
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的基础上筛选,满足Min(salary)>①
SELECT MIN(salary)
FROM employees
GROUP BY department_id
HAVING MIN(salary) >(SELECT MIN(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
/*
注意:下面sql中的IN 可以用Any来替换
如果为Not in 则可以 用 <>ALL 来替换
*/
SELECT last_name
FROM employees
WHERE department_id IN(SELECT DISTINCT department_id FROM departments
WHERE location_id IN(1400,1700)
);
#案例2:返回其他工种中比job_id为‘IT_PROG’部门任一工资低的员工的:工号、姓名、job_id以及salary
SELECT employee_id,last_name,job_id,salary
FROM employees
WHERE salary < ANY(SELECT salary
FROM employees
WHERE job_id = 'IT_PROG')
AND job_id <>'IT_PROG';
#或
SELECT employee_id,last_name,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:返回其他工种中比job_id为‘IT_PROG’部门所有工资都低的员工的:工号、姓名、job_id以及salary
SELECT employee_id,last_name,job_id,salary
FROM employees
WHERE salary < ALL(SELECT salary
FROM employees
WHERE job_id = 'IT_PROG')
AND job_id <>'IT_PROG';
SELECT employee_id,last_name,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 =(SELECT MIN(employee_id) FROM employees)
AND salary = (SELECT MAX(salary) FROM employees);
#用行子查询来实现
SELECT *
FROM employees
WHERE (employee_id,salary)=(
SELECT MIN(employee_id),MAX(salary)
FROM employees)