select max(dh) from (select dh from a union select dh from b union select dh from c) d
我开始想的和lys1706228一样,但语法错误。 libin_ftsafe(子陌红尘) 的正确。
听说下面的语句可以在一定情况下有所优化,也不知道是否真的,即在最小结果集当中找结果.select max(dh) as dh from (select max(dh) as dh from a union all select max(dh) as dh from b union all select max(dh) as dh from c )
select max(dh) as dh from (select max(dh) as dh from a union all select max(dh) as dh from b union all select max(dh) as dh from c ) a
libin_ftsafe(子陌红尘) 的正确。
(select max(dh) as dh from a
union all
select max(dh) as dh from b
union all
select max(dh) as dh from c
)
(select max(dh) as dh from a
union all
select max(dh) as dh from b
union all
select max(dh) as dh from c
) a