Question

I am doing a inner join between two tables where one is an association table, so there is a many to one relationship. I am trying to come up with a query that can decide if the key on the join exist more than once than store a value multiple in the update column, but not sure the efficient way to make this happen:

SELECT 
  MainTable.Name 
FROM MainTable 
INNER JOIN ASSN_Main ON MainTable.AppID = ASSN_Main.AppID 
WHERE 
  EXISTS (SELECT 
            COUNT(MainTable.AppID) 
          FROM MainTable 
          INNER JOIN ASSN_Main ON MainTable.AppID = ASSN_Main.AppID 
          GROUP BY 
            MainTable.AppID 
          HAVING 
            (COUNT(MainTable.AppID)>1));

The problem is the subquery grabs the correct ones that have duplicates on the appid, but the main SELECT query grabs all appid names instead of only the ones that exist in the subquery. Not sure whats going wrong since the subquery is correct?

Answer 1

There is no relation between the items in the main query and the items in the subquery, so what the query is returning is “all items, if there are any items that have duplicates”. What you want is “all items where there are duplicates”:

SELECT 
  MainTable.Name 
FROM MainTable m
INNER JOIN ASSN_Main a ON m.AppID = a.AppID 
WHERE 
  EXISTS (SELECT AppID
          FROM ASSN_Main
          WHERE AppID = m.AppID
          GROUP BY AppID 
          HAVING COUNT(*)>1);