求一SQL语句优化```
我表sub中有一个类别字段sortid，有个日期字段lastback,我现在想取每个类别的最新5条数据。然后再取前20条。
select top 20 * from (
select top 5 * from sub where isnull(isback,1)=0 and sortid=1
union all
select top 5 * from sub where isnull(isback,1)=0 and sortid=2
union all
select top 5 * from sub where isnull(isback,1)=0 and sortid=3
union all
select top 5 * from sub where isnull(isback,1)=0 and sortid=4
union all
select top 5 * from sub where isnull(isback,1)=0 and sortid=5
union all
select top 5 * from sub where isnull(isback,1)=0 and sortid=6
union all
select top 5 * from sub where isnull(isback,1)=0 and sortid=7
union all
select top 5 * from sub where isnull(isback,1)=0 and sortid=9) a
order by lastback desc
但目前这方法好象太太太笨了。 。所以想优化一下。。请教高手。谢谢`！
[解决办法]
select * from sub a where priamyKey in(select top 5 priamyKey from sub where sortid=a.sortid order by lastback desc)
order by sortid
priamyKey为主键.
