A表中有 字段 a, b,c。现在要判断a字段的值不能重复、、求sql语句

查找a不重复的数据 select a,b,c from 表 where a not in ( select a from 表 group by a having count(a) > 1 ) 查找a重复的数据 select a from 表 group by a having count(a) > 1