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。