Question

I’m currently working on a proprietary shopping cart system and was having a few problems with getting products out with the correct pricing.

Basically my table structure is as follows:

Products table: (Only relevant columns are represented)

----------------------------------------------------
productid | product | descr | disporder| list_price|
----------------------------------------------------
    1        name      desc      1          0.00
    2        name      desc      4          0.00
    3        name      desc      2          2.45

Pricing table:

----------------------------------------
priceid | productid | price | variantid|
----------------------------------------
    1         1       13.91      1
    2         2       54.25      4
    3         2       47.23      2

Variants Table:

-------------------------------
variantid | productid | active|
-------------------------------
    1          1          Y     
    2          2          Y
    3          2          Y

So, each product can have – and in most cases does have – multiple variants. My current SQL query I have managed to create thus far is:

    SELECT 
        products.productid, product, descr, p.price, i.image_path
    FROM 
        products
    LEFT JOIN 
        pricing AS p 
    ON  
        p.variantid = (SELECT variantid FROM variants 
            WHERE productid = products.productid LIMIT 1)
    LEFT JOIN 
        images_T AS i 
    ON
        i.id = products.productid
    GROUP BY
        products.productid
    ORDER BY
        products.disporder

However, my problem arises when a product does not have a variant. If a product does not have a variant associated with it, the price will be in the list_price column of the products table. How would I go about performing a check to see if a product does indeed have a variant. If not, it should effectively bypass the variants table and get the pricing from list_price within the products table.

Answer 1

Yes, CASE is an option, or COALESCE:

SELECT 
    products.productid, product, descr, 
    COALESCE(products.list_price, p.price) AS price, 
    i.image_path
...

Just join both prices and when the first is NULL the other will be selected.