mysql 如何在派生表上放置别名

Question

我有以下疑问

SELECT * FROM(
    (select * from `responses` a  order by id, exam, question)
       left JOIN
          (select max(exam) as maxexam from responses b )
              on a.exam <= b.maxexam
              and id > 2) c 

我收到以下错误消息

Every derived table must have its own alias

Answer 1

如果您使用连接条件

on a.exam <= b.maxexam and id > 2

，那么您需要为两个子查询提供别名

a

和

b

：

SELECT * FROM
    /* alias a applied to first subquery: */
    (select * from `responses`  order by id, exam, question) a
    left JOIN
    /* alias b applied to second subquery: */
    (select max(exam) as maxexam from responses) b
    on a.exam <= b.maxexam and id > 2
;