sql-server多字段去重

sql语句是从右到左执行的←
优化原则：能迅速排查的语句放在右边

显示部门平均工资低于2000的部门号和平均工资，这里不用子查询而是用having

--having往往和group by结合使用，对分组查询的结果进行筛选
select avg(sal)  avg_sal,deptno from emp
group by deptno having avg(sal) < 2000

--group by having order by同时出现的顺序
select avg(sal)  avg_sal,deptno from emp
group by deptno 
having avg(sal) < 2000
order by avg(sal)

--上面介绍过执行顺序right-to-left，所以下面这个是不行的
select avg(sal)  avg_sal,deptno from emp
group by deptno having avg_sal < 2000

--在选择列中如果有列，表达式，和分组函数，那么这些列和表达式必须有一个出现在
--group by子句中，否则出错
--1、没有group by
select avg(sal)  avg_sal,deptno from emp having avg_sal < 2000
--2、group by中的内容不是select中的
select avg(sal)  avg_sal,deptno from emp
group by user_id having avg_sal < 2000

sqlserver不支持多字段in not-in

就像：

delete from user_geo_simplified
where (user_id,user_gor_simplified) in (select [user_id],user_gor_simplified from #temp1)
and timeDate not in (select max(timeDate) from user_geo_simplified group by[user_id],user_gor_simplified having count(*)>1)

但是可以这样：

Select a.user_id ,a.geo,Max(a.timeDate) as timeDate INTO USER_GEO_LEFT_5 
from (SELECT user_id ,geo ,timeDate FROM user_geo_simplified) a 
group by a.user_id ,a.geo 
order by USER_ID,timeDate
SELECT * FROM USER_GEO_LEFT_5

事实上group by子句的执行总是在当前group内

--查询每个部门的平均工资
select avg(sal) as avg_sal, deptno from emp
group by deptno

select distinct item_id into #temp from train_item

select [tianchi].[dbo].[train_user].user_id,
       [tianchi].[dbo].[train_user].item_id,
       [tianchi].[dbo].[train_user].behavior_type,
       [tianchi].[dbo].[train_user].user_geohash,
       [tianchi].[dbo].[train_user].timeDate

into train_user_on_certain_item
from train_user, #temp
where train_user.item_id = #temp.item_id

alter table item_category add aa bigint
update item_category set aa = item_category 
alter table item_category drop column aa

以上是一些基本操作的例子

/*查看列*/
select * from information_schema.columns where table_name = '表名' 

/*添加列*/
alter table 表名 add 列名 varchar(55)

/*删除列*/
alter table 表名 drop column 列名

/*修改列名称*/
exec sp_rename '表名.字段名' , '新名', 'column'

/*修改列类型*/
alter table 表名 alter column 列名 varchar(22)