union all 与 union 去重

union all与union的区别：
union all对结果集不会去除重复的结果，union会去除重复的结果

select distinct HZDDH,HTBH,HTSX,XSYMC,b.PH,b.YS from XSHTMLH h left join XSHTMLB b on h.djlsh=b.djlsh
union all
select distinct  HZDDH,HTBH,ddlx as HTSX,YWY as XSYMC,b.PH,b.ys from CPCJDDH h left join CPCJDDB b on h.djlsh=b.djlsh

等价于

select HZDDH,HTBH,HTSX,XSYMC,b.PH,b.YS from XSHTMLH h left join XSHTMLB b on h.djlsh=b.djlsh
union 
select HZDDH,HTBH,ddlx as HTSX,YWY as XSYMC,b.PH,b.ys from CPCJDDH h left join CPCJDDB b on h.djlsh=b.djlsh