Question

I have this query, but I want to only join PostageRules and PostageGroups if p.delivery has a value of -1

To clarify, I only want one delivery value, if p.delivery is -1 then we need to get the po.delivery value from the PostageRules table. At the moment the query gets the po.delivery regardless of the value of p.delivery

Is this possible to achieve in a single query?

SELECT  
   p.id as prod_id, p.delivery as delivery, 
   po.delivery AS delivery 
FROM 
   products AS p 
LEFT JOIN 
   PostageRules AS po ON p.shopkeeper = po.shopkeeper
LEFT JOIN 
   PostageGroups  AS pg ON po.groupID = pg.id
        AND po.minQty <= 1
        AND po.maxQty >= 1
        AND po.minPrice <= p.Price
        AND po.maxPrice >= p.Price
        AND po.minWeight <= p.weight
        AND po.maxWeight >= p.weight                
WHERE 
    (p.id = '32323') 
ORDER BY 
    po.preference   

Answer 1

SELECT  p.id as prod_id, ISNULL(po.delivery, p.delivery) AS delivery 
    FROM products AS p 
    LEFT JOIN PostageRules AS po ON p.shopkeeper = po.shopkeeper and p.delivery = -1
    LEFT JOIN PostageGroups  AS pg ON po.groupID = pg.id and p.delivery = -1
    AND po.minQty <= 1
    AND po.maxQty >= 1
    AND po.minPrice <= p.Price
    AND po.maxPrice >= p.Price
    AND po.minWeight <= p.weight
    AND po.maxWeight >= p.weight                
    WHERE (p.id = '32323') ORDER BY po.preference