sql
1 as
as的作用是设置别名，可以用于字段也可以用于表：
select id as i,age as a from student as t1;
用于字段时，where、having中不能用别名，但order和group中可以.
-- 错误
select id as i from student where i>1;
-- 正确
select id as i from student group by i order by i;
用于表时，一般单个表没必要，所以常用于关联查询
select * from student as t1 join claz as t2
on t1.clazid=t2.id
2 having
having和where都是接条件的，不过having是只能用在group by之后表示分组后的条件
select count(*) as count,clazid from student where age>10
group by clazid having clazid>1;
这里的where和having条件框定的内容不同，where用于限制student群体，而having用于限制分组后的count clazid组成的集合。