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

或

SELECT department_name
FROM departments
WHERE department_id IN(
	SELECT department_id
	FROM employees
);

#查询没有女朋友的男神信息

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

或

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