MySQL(九)DQL之子查询

一、含义

嵌套在其他语句内部的select语句称为子查询或内查询,外面的语句可以是insert、update、delete、select等,一般select作为外面语句较多,外面如果为select语句,则此语句称为外查询或主查询。

二、分类

2.1、按出现位置

select后面:仅仅支持标量子查询

from后面: 表子查询

where或having后面: 标量子查询、列子查询、行子查询

exists后面:标量子查询、列子查询、行子查询、表子查询

2.2、按结果集的行列

标量子查询(单行子查询):结果集为一行一列

列子查询(多行子查询):结果集为多行一列

行子查询:结果集为多行多列

表子查询:结果集为多行多列

三、示例

3.1、where或having后面

案例1:谁的工资比 Abel 高

SELECT * FROM employees WHERE salary>(SELECT salary FROM employees WHERE last_name = 'Abel');

案例2: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 MIN(salary),department_id FROM employees GROUP BY department_id HAVING MIN(salary)>( SELECT MIN(salary) FROM employees WHERE department_id = 50 );

3.2、select后面

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;

3.3、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

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;

3.4、exists后面

语法:exists(完整的查询语句)

结果:1或0

案例1:查询有员工的部门名

SELECT department_name FROM departments d WHERE EXISTS( SELECT * FROM employees e WHERE d.`department_id`=e.`department_id`);

相当于

SELECT department_name FROM departments d WHERE d.`department_id` IN( SELECT department_id FROM employees )

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