select a,max(b),min(b)
from
(
select a,b from tablename
union all
select a,c from tablename
union all
select a,d from tablename
) a
group by a
from
(
select a,b from tablename
union all
select a,c from tablename
union all
select a,d from tablename
) a
group by a
Select min(b) b From Tablename --求最小
其原理同上
FROM(
select a,最大值=max(b),最小值=min(b) from TABLENAME group by a
union
select a,最大值=max(c),最小值=min(c) from TABLENAME group by a
union
select a,最大值=max(d),最小值=min(d) from TABLENAME group by a
)a
group by a