SQL报错Every derived table must have its own alias

前言：在使用mysql做count计数多表查询或子查询的时候犯了一个低级的错误：
1248 - Every derived table must have its own alias（每个派生表都必须有自己的别名）

报错sql:

SELECT
	count(*) 
FROM
	(
	SELECT
		t.id id,
		t1.id t1_id,
		t2.id t2_id 
	FROM
		t_school t
		LEFT JOIN t_teacher t1 ON t.teacherid = t1.id
		LEFT JOIN t_user t2 ON t.userid = t2.id 
	WHERE
	1 = 1 
	) 

执行这条语句就会报错：Every derived table must have its own alias。

简单的SQL构成是：select 字段名 from 表名，上面子查询里面查出来的是数据，没有被当成一个表，使用我们只需要在SQL后面加上AS 表名 给他一个表的名字即可

修改过后的SQL：

SELECT
	count(*) 
FROM
	(
	SELECT
		t.id id,
		t1.id t1_id,
		t2.id t2_id 
	FROM
		t_school t
		LEFT JOIN t_teacher t1 ON t.teacherid = t1.id
		LEFT JOIN t_user t2 ON t.userid = t2.id 
	WHERE
	1 = 1 
	) AS a

加上表名之后即可解决报错了。