mysql order by field name contained in another table field value

Question

I have a search query where i need to order by a field contained in another table field value. Not very clear perhaps :)

Consider this :

        $requete = "
 SELECT A.*,
 (SELECT payspan FROM Categories WHERE id=A.category_id) AS SortingFieldName 
 FROM Annonces A 
 JOIN Categories C ON C.id=A.category_id
 WHERE A.deleted=0
 ORDER BY SortingFieldName";

I would like to order like this, but it does not work : ORDER BY A.SortingFieldName

SortingFieldName will contain the name of the field, but not the value of that field in the Annonce table.

How can i get the value of that field and then order by ?

I tried a variable, but dont seem to be able to use a variable inside a query ...

To make it simple, i would like to order Annonces lines by the field configured in Categories.

Like, if i have in Annonces : id,name,category_id,price_day,price_month

And in Categories : id,name,pricefield

I would like to order lines in Annonce with the value of price_month or price_day according to the configuration of the category.

Thx for any help.

Answer 1

Use this:

ORDER BY CASE SortingFieldName
            WHEN 'field1' THEN field1
            WHEN 'field2' THEN field2
            WHEN 'field3' THEN field3
            ...
         END

Replace field1 , field2 , etc. with the actual field names.

If you can't list all the possible field names, you'll have to write use dynamic SQL.