mysql exists子查询

exists后面的子查询,exists 只关心有没有，返回布尔类型

#案例：查询有员工的部门名

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

或

SELECT department_nameFROM departmentsWHERE department_id IN(    SELECT department_id    FROM employees);

#查询没有女朋友的男神信息

SELECT bo.*FROM boys boWHERE  NOT EXISTS(    SELECT b.boyfriend_id    FROM beauty b    WHERE b.`boyfriend_id`=bo.`id`);

或

SELECT bo.*FROM boys boWHERE bo.`id` NOT IN(    SELECT boyfriend_id    FROM beauty b);