mysql用多表查询注意

mysql多表查询

在用mysql去用多表查询的时候，经常会报一个错

every derived table must have its own alias

这个告诉我们就是在用多表查询的时候要给每个取到的表给个自己的别名。


//这段sql代码在mysql会报错
select max(enrollment),min(enrollment)
from(
select sec_id,semester,year,count(distinct id)as enrollment
from takes
group by sec_id,semester,year
)
//给子查询返回的新关系取个别名，我随意叫个myalias。
select max(enrollment),min(enrollment)
from(
select sec_id,semester,year,count(distinct id)as enrollment
from takes
group by sec_id,semester,year
)as myalias;