Now in the database has multiple items need to be merged, then one of the values when the current or previous, if for the two values are returned, the current, then return to the previous, if only the previous don't exist returns an empty:
Before the practice of the query out the value of the merger, when need to change now:
The select orbis group_concat (distinct dmcdate separator '$$') from orbis group by bvdnum, dmbnum;