SQL语句 里的order by 、group by、having、where 用在哪里

Question

SQL语句 里的order by 、group by、having、where 应该用在哪里 老混起来特别是having和where 两个什么区别啊

Answer 1

orderby 是排序 group by 用于分组查询 having 和where 区别在于 where 是用在分组前筛选 having 用于分组后筛选

Answer 2

where是用在你对条件的限制，当你使用group by是一定要使用having，这个是按照什么分组，order by是排序，你检索出来的记录按照什么顺序排列，比如，字母的先后顺序啊，数字的大小啊等等

Answer 3

select ziduan1,ziduan2,ziduan3 from biao order by ziduan1 --从小到大
select ziduan1,ziduan2,ziduan3 from biao order by ziduan1 desc --从大到小
select ziduan1,ziduan2,ziduan3 from biao group by ziduan1,ziduan2,ziduan3 --分组可以去除重复值
select ziduan1,sum(ziduan2) as 2,sum(ziduan3) as 3 from biao group by ziduan1 --非计算的字段需要分组
select count(ziduan1) 1 from biao group by ziduan1 --可以统计数据条目数
select count(ziduan1) 1 from biao where ziduan1=ziduan2 group by ziduan1 --可以统计ziduan1=ziduan2的数据条目数

select ziduan1 --查询的字段
from biao --表名称
where ziduan1<>'' --字段1不等于空
group by ziduan1 --按字段1分组
having sum(ziduan2)=sum(ziduan3) --字段2合计等于字段3合计
order by ziduan1 --排序字段

where 条件是字段直接判断，没有聚合条件
比如：where shuliang<>0 and shuliang =1 and1+1=2 等等
having 是聚合条件
比如：having sum(shuliang)<>0 and sum(shuliang)=1 and sum(1+1)=2等等