三个变量取最大值: select prd_itm,(case when prd_p3qty>(case when prd_p1qty>prdp2qty then prd_p1qty else prd_p2qty end) then prd_p2qty else (case when prd_p1qty>prdp2qty then prd_p1qty else prd_p2qty end) end) as 最高月用量 from prd_det --或: select prd_itm,max(prd_tmp) as 最高月用量 from( select prd_itm,prd_tmp=prd_p1qty from prd_det union all select prd_itm,prd_tmp=prd_p2qty from prd_det union all select prd_itm,prd_tmp=prd_p3qty from prd_det ) tmp group by prd_itm
select prd_itm,(case when prd_p3qty>(case when prd_p1qty>prdp2qty then prd_p1qty else prd_p2qty end) then prd_p2qty else (case when prd_p1qty>prdp2qty then prd_p1qty else prd_p2qty end) end) as 最高月用量
from prd_det
--或:
select prd_itm,max(prd_tmp) as 最高月用量 from(
select prd_itm,prd_tmp=prd_p1qty from prd_det
union all
select prd_itm,prd_tmp=prd_p2qty from prd_det
union all
select prd_itm,prd_tmp=prd_p3qty from prd_det ) tmp group by prd_itm