093-exists后子查询，可以用in代替

exists返回结果为一个Boolean值

SELECT EXISTS(SELECT employee_id FROM employees );

#查询有员工名的部门名

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

先执行主句select
然后执行where的条件语句
上句可以用一下执行

SELECT `department_name`
FROM `departments` d
WHERE d.`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`
);

等价于

SELECT bo.*
FROM `boys` bo
WHERE bo.`id` IN(
	SELECT `boyfriend_id`
	FROM `beauty`
);