Question

I have following data

 GROUP_ID   INDEX
   NULL       1
     1        2
   NULL       3
     1        4
   NULL       5
     2        6
     2        7

What I want is to get a field that contains:

• if group_id IS NOT NULL, a smallest index in that group
• if group_id IS NULL, then index from that row

The result dataset for the above example would be:

 GROUP_ID   INDEX  GROUP_INDEX_MIN
   NULL       1          1
     1        2          2
   NULL       3          3
     1        4          2
   NULL       5          5
     2        6          6
     2        7          6

In Oracle I would resolve this by using (CASE WHEN group_id IS NOT NULL THEN MIN(index) OVER (PARTITION BY group_id) ELSE group_id END), but since MySQL does not support that, I really don’t know how to proceed 🙂 I can solve this by using a subquery that retrieves minimal values for each group and then left joining to it, but I think that there must be a more elegant solution.

If you need any more information, please ask.
Thanks.

Answer 1

Just create an inline view representing the min Group_index_min (minT) and then join to it and do a coalesce between group_ID and the min

SELECT t.group_id, 
       t.index, 
       Coalesce(t.group_id, minTgroup_index_min) group_index_min 
FROM   yourtable t 
       LEFT JOIN (SELECT group_id, 
                         MIN(index) group_index_min 
                  FROM   yourtable
                  GROUP BY 
                         group_id) minT 
         ON t.group_id = minT.group_id