group by 如何合并字符串优化记？

sqlserver 2005及以上版本

表(tb)

id    value

1     aa

2     cc

3     bb

3     dd

4     aa

4     cc

4     dd

5     aa

优化前：

select id, value = (select value+',' from tb as rtb where rtb.id = a.tb.id for xml path(''))

from tb as a

group by a.id

优化后：（思路：将count(0)数等于1或者等于2的记录，通过min,max处理，减少每个id都要查询表）

select id, value =case count(0) when 1 then min(value)

when 2 then min(value)+','+max(value)

else   (select value+',' from tb as rtb where rtb.id = a.tb.id for xml path('')) end

from tb as a

group by a.id