Question

Please I have the following tables

Field
+----+--------+----------+---------------------+
| id | name   | Type     | date                |
+----+--------+----------+---------------------+
|  1 |  price | int      | 2009-09-02 00:07:07 |
|  2 |  width | float    | 2009-09-02 00:07:08 |
|  3 |  height| float    | 2009-09-02 00:07:30 |
+----+--------+--------------------------------+


Post
+----+--------+---------------------------------+
| id |title   | Body                            |
+----+--------+---------------------------------+
|  1 |  test  | Bla bla bla                     |
|  2 | Hello  | Bla bla bla                     |
|  3 | Its me | Bla bla bla                     | 
+----+--------+---------------------------------+


Post_Annex
+----+--------+----------+---------------------+
| id | IdPost | IdField  | Value               |
+----+--------+----------+---------------------+
|  1 |  1     | 1        | 50                  |
|  2 |  1     | 2        | 21                  |
|  3 |  1     | 3        | 3                   |
|  4 |  2     | 1        | 10                  |
|  5 |  2     | 2        | 2                   |
|  6 |  2     | 3        | 150                 |
|  7 |  3     | 1        | 1                   |
|  8 |  3     | 2        | 25                  |
|  9 |  3     | 3        | 15                  |
+----+--------+--------------------------------+

And I desire to select data from the Post and Post_Annex tables, group them by the Id of the Post table and order them by the price (this mean by IdField of the Post_Annex table when its equal to 1). So this is my query :

SELECT p.title
    FROM Post p
    LEFT JOIN Post_Annex pa
       ON p.id = pa.IdPost

    GROUP BY p.id

    ORDER BY case when pa.IdField = 1 then 0 else 1, pa.Value DESC

This gave me the result in this order : its Me | Hello | test

This is fine. But even if I try the order by the width (IdField = 2) or height (IdField = 3) it gave me the same result.

Please masters any Idea to help me resolving this ? Thanks in advance.

Answer 1

I would just modify the join clause, to specify which field you want to sort:

SELECT p.title
FROM Post p
     LEFT JOIN Post_Annex pa
     ON p.id = pa.IdPost
        AND pa.idfield=1
ORDER BY pa.Value DESC

and then you don’t have to group by p.id.

Your query doesn’t always work because you are grouping by p.id but you are selecting (and ordering by) other nonaggregated columns, and the values of those columns can be undetemined.