Question

I am attempting to join two tables (purchases and purchase attributes), and then group based on the join, however NULL values are generated during the join as not every entry in table A has a corresponding ‘attributes’ in table B – this then kills my ‘group by’.

My actual implementation is more complicated than this but here is a trimmed concept of what I am trying to achieve:

I have the following Tables:

purchases:

|purchase_id | products_id |   qty    |
|      1     |   10        |   100    |
|      2     |   11        |   100    |
|      3     |   10        |   300    |
|      4     |   12        |    50    |

purchase_attributes:

|purchase_attributes_id | purchase_id | options_id |
|         1             |      1      |     5      |
|         2             |      3      |     5      |
|         3             |      4      |     5      |

Now, my goal is to calculate the total qty of a particular product, grouping by options_id.
So, I have the following sql:

select products_id, options_id, SUM(qty) 
from purchases
left join purchase_attributes using(purchase_id)
group by products_id, options_id

Which works for all cases where the purchase_id is listed in the purchase_attributes table, however for the case of purchase_id 2, which does not appear in purchase_attributes, it does not display in my result set.

Example result set:

| products_id  | options_id  |  SUM(qty)  |
|     10       |     5       |    400     |
|     12       |     5       |     50     |

However I would like to receive the following result set:

| products_id  | options_id  |  SUM(qty)  |
|     10       |     5       |    400     |
|     11       |     NULL    |    100     |
|     12       |     5       |     50     |

I have tried a few things, including Coalesce inside the group by and Union(ing) two select statements together, however I cannot seem to get the correct results.

Any ideas? thanks in advance.

Answer 1

This does produce exactly your desired results.

SELECT products_id,options_id,SUM(qty)
FROM   (SELECT 1 AS purchase_id,10 AS products_id,100 AS qty
        UNION ALL
        SELECT 2,11,100
        UNION ALL
        SELECT 3,10,300
        UNION ALL
        SELECT 4,12,50) purchases
       LEFT JOIN (SELECT 1 AS purchase_attributes_id,1 AS purchase_id,5 AS
                         options_id
                  UNION ALL
                  SELECT 2,3,5
                  UNION ALL
                  SELECT 3,4,5) purchase_attributes USING(purchase_id)
GROUP  BY products_id,options_id  

Please provide an example that actually demonstrates your issue.