Question

I’ve a table with values like:

row   item_id   device_token
1     1         1234
2     2         2345
3     1         
4     2         
5     1         
6     1         1234

I would like to select all values, but group them by item_id and device_token.

SELECT item_id, device_token FROM hits GROUP BY item_id, device_token

Grouping the above data would results in 4 rows, as row 1,6 and 3,5 would be combined. Can I make a query which does not group the rows where the device_token is empty/null (row 3 and 5)?

Answer 1

If you don’t want the rows where device_token is empty this is easy – just use a simple WHERE clause as others have suggested.

And if you want to have both, those with matching device_token grouped and those without a device token as single rows, you can combine two queries with a UNION:

SELECT item_id, device_token 
  FROM hits 
  WHERE device_token IS NOT NULL 
  GROUP BY item_id, device_token 
UNION ALL
SELECT item_id, device_token 
  FROM hits 
  WHERE device_token IS NULL