mysql - 为什么每个派生表必须有自己的别名？

Question

select a as average,name 
from (select avg(marks) as a,name 
      from marks,student where rollno=roll group by marks.roll);

ERROR 1248 (42000): Every derived table must have its own alias

我知道更正。只是想知道为什么我必须使用别名？

select a as average,name 
from (select avg(marks) as a,name 
      from marks,student where rollno=roll group by marks.roll)xxx;

Answer 1

这只是 MySQL 设计中的一个怪癖。在这种情况下，Oracle 和 SQL-Server 不需要别名。