MySql 1248 - Every derived table must have its own alias

执行一个sql语句，报错：1248 - Every derived table must have its own alias

提示说每一个衍生出来的表，必须要有自己的别名

执行子查询的时候，外层查询会将内层的查询当做一张表来处理，所以我们需要给内层的查询加上别名

 错误的：

select count(*)
from(
    select
    FROM_UNIXTIME(`user_task`.`CREATE_TIMESTAMP`, "%Y-%m-%d")
    FROM
    r_user_task `user_task`
    LEFT JOIN
    e_task `task` ON `user_task`.`TASK_ID` = `task`.`ID`
    where `user_task`.`COMPLETE_STATUS` = "" and `user_task`.`user_id` =
    group by FROM_UNIXTIME(`user_task`.`CREATE_TIMESTAMP`, "%Y-%m-%d")
) 

正确的sql，应该加上别名，如下：

select count(*)
from(
    select
    FROM_UNIXTIME(`user_task`.`CREATE_TIMESTAMP`, "%Y-%m-%d")
    FROM
    r_user_task `user_task`
    LEFT JOIN
    e_task `task` ON `user_task`.`TASK_ID` = `task`.`ID`
    where `user_task`.`COMPLETE_STATUS` = "" and `user_task`.`user_id` =
    group by FROM_UNIXTIME(`user_task`.`CREATE_TIMESTAMP`, "%Y-%m-%d")
) as t

参考：

https://blog.csdn.net/cao478208248/article/details/28122113