SQL 如何实现两行记录合并

Question

想实现按原表中的前面两个列相同时，把第三个列求和，第四个列 合并

Answer 1

select SCDDCP_LYLSBH,SCDDCP_LYFLBH,sum(JHSL) as JHSL
, XSTD=(select XSTD+',' from a as a2
where a2.SCDDCP_LYFLBH=a1.SCDDCP_LYFLBH and a2.SCDDCP_LYFLBH=a1.SCDDCP_LYFLBH for xml path(''))
from a as a1
group by SCDDCP_LYLSBH,SCDDCP_LYFLBH