送给sql新手查询表重复行

1 select custcode, custname from bd_cubasdoc where custname in (select custname from bd_cubasdoc 
group by  custname having count(*)>=2)
 
2 select  custname from bd_cubasdoc group by custname having count(*)>=2

3 select custcode,custname from bd_cubasdoc group by custcode,custname having count(*)>=2

1,2可以查询到，3不可以。