select t.id,t.name,(case when u.dt is null then t.dt else u.dt end)as updatedt from testta t left join
(select id,max(dt) as dt from updatedate group by id) as u on t.id=u.id
dt是时间
删除列：alter table testta drop column dt2
添加列及默认值：alter table testta add dt2 datetime constraint dt2 （default getdate() with values）
哪一月的每一天的数量是：
select convert(char(11),dt2),count(id) from testta where dt2 between convert(datetime,'04 01 2006') and convert(datetime,'05 01 2006')
group by convert(char(11),dt2)
convert(char(11),dt2)：只取年月日
分页语句：
SELECT TOP pagesize * FROM articles
WHERE id NOT IN ( SELECT TOP ( pagesize * pageindex ) id FROM articles ORDER BY id DESC )
ORDER BY id DESC
pagesize：每页显示几条
pageindex：第几页
replace(字段名，char(13),'') 将字段中的回车符替换成为空
