SELECT column_a,column_b,COUNT(*) as num FROM TABLE GROUP BY column_a,column_b;
SELECT column_a,column_b,COUNT(*) as num FROM TABLE group by column_a,column_b having count(*)>1;
查询表中 A/B 字段联合是否唯一
原文：http://blog.51cto.com/13523022/2064563
