Sql group by与order by 后面的列的问题

Question

在我的student表中有sno,sname,sdept,sage
我直接这样写order by后面的列并没有在select后面答案是对的
select sno,sname from student order by sage desc; //没有问题
但是我用加上group by后，按照它报的错，order by后面的列必须要是在group by中出现的，这是为什么呢？
select sname,sdept,ssex from student group by sname,sdept,ssex having sdept ='MA' and ssex = '女' order by sage; //有错
select sname,sdept,ssex from student group by sname,sdept,ssex having sdept ='MA' and ssex = '女' order by sname; //这样也没问题

Answer 1

这是group by的语法，分组了以后，order需要是分组的列