Question

I’ve searched for an answer on this but can’t find quite how to get this distinct recordset based on a condition. I have a table with the following sample data:

Type    Color   Location    Supplier
----    -----   --------    --------
Apple   Green   New York    ABC
Apple   Green   New York    XYZ
Apple   Green   Los Angeles ABC
Apple   Red     Chicago     ABC
Apple   Red     Chicago     XYZ
Apple   Red     Chicago     DEF
Banana  Yellow  Miami       ABC
Banana  Yellow  Miami       DEF
Banana  Yellow  Miami       XYZ
Banana  Yellow  Atlanta     ABC

I’d like to create a query that shows the count of unique locations for each distinct Type+Color where the number of unique locations is more than 1, e.g.

Type    Color   UniqueLocations
----    -----   --------
Apple   Green   2
Banana  Yellow  2

Note that {Apple, Red, 1} doesn’t appear because there is only 1 location for red apples (Chicago). I think I’ve got this one (but perhaps there is a simpler method). I’m using:

SELECT Type, Color, Count(Location) FROM
(SELECT DISTINCT Type, Color, Location FROM MyTable)
GROUP BY Type, Color HAVING Count(Location)>1;

How can I create another query that lists the Type, Color, and Location for each distinct Type,Color when the count of unique locations for that Type,Color is greater than 1? The resulting recordset would look like:

Type    Color   Location
----    -----   --------
Apple   Green   New York
Apple   Green   Los Angeles
Banana  Yellow  Miami
Banana  Yellow  Atlanta

Note that Apple, Red, Chicago doesn’t appear because there is only 1 location for red apples. Thanks!

Answer 1

Use a COUNT(DISTINCT Location) and join against a subquery on Type and Color The GROUP BY and HAVING clauses as you have attempted to use them will do the job.

/* Be sure to use DISTINCT in the outer query to de-dup */
SELECT DISTINCT
   MyTable.Type,
   MyTable.Color,
   Location
FROM 
  MyTable
  INNER JOIN (
    /* Joined subquery returns type,color pairs having COUNT(DISTINCT Location) > 1 */
    SELECT
      Type,
      Color,
      /* Don't actually need to select this value - it could just be in the HAVING */
      COUNT(DISTINCT Location) AS UniqueLocations
    FROM
      MyTable
    GROUP BY Type, Color
    /* Note: Some RDBMS won't allow the alias here and you 
       would have to use the expanded form
       HAVING COUNT(DISTINCT Location) > 1
     */
    HAVING UniqueLocations > 1
  /* JOIN back against the main table on Type, Color */
  ) subq ON MyTable.Type = subq.Type AND MyTable.Color = subq.Color

Here is a demonstration