子查询

例题：比较谁的工资比Abel高

#方式一
SELECT salary 
FROM employees 
WHERE last_name = 'Abel'; 

SELECT last_name,salary 
FROM employees 
WHERE salary > 11000;
#方式二：自连接 
SELECT e2.last_name,e2.salary 
FROM employees e1,employees e2 
WHERE e1.last_name = 'Abel' 
AND e1.`salary` < e2.`salary`

#方式三：子查询 
SELECT last_name,salary 
FROM employees 
WHERE salary > ( SELECT salary FROM employees WHERE last_name = 'Abel' );
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18

子查询的基本使用

子查询（内查询）在主查询之前一次执行完成。
子查询的结果被主查询（外查询）使用 。
注意事项：

子查询要包含在括号内
将子查询放在比较条件的右侧
单行操作符对应单行子查询，多行操作符对应多行子查询

子查询的分类

我们按内查询是否被执行多次，将子查询划分为 相关(或关联)子查询 和 不相关(或非关联)子查询。

单行子查询

单行比较操作符

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);
1
2
3
4

返回公司工资最少的员工的last_name,job_id和salary

SELECT last_name, job_id, salary 
FROM employees 
WHERE salary = (SELECT MIN(salary) FROM employees);
1
2
3

HAVING 中的子查询

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

SELECT department_id, MIN(salary) 
FROM employees 
GROUP BY department_id 
HAVING MIN(salary) > (SELECT MIN(salary) FROM employees WHERE department_id = 50);
1
2
3
4

CASE中的子查询

显式员工的employee_id,last_name和location。其中，若员工department_id与location_id为1800 的department_id相同，则location为’Canada’，其余则为’USA’。

SELECT employee_id, last_name, 
	(CASE department_id WHEN (SELECT department_id FROM departments WHERE location_id = 1800) 
THEN 'Canada' ELSE 'USA' END) location 
FROM employees;
1
2
3
4

子查询中的空值问题

SELECT last_name, job_id 
FROM employees 
WHERE job_id = (SELECT job_id FROM employees WHERE last_name = 'Haas');#不返回任何值
1
2
3

多行子查询

多行比较操作符