Question

My first table is an item table. (invoice_inventory)
My second table is the pricelist (inventory_prices), there I have a default price list (company_id = 0) and several special price lists (but not for every company).

With the following query I try to get all items with their prices for the company_id = 2.
It should use the special price list (company_id = 2 if it exists) else it should use the default price (company_id = 0).

I thought by GROUPING + ORDERING it would work, but I forgot that GROUP BY gets executed before ORDER.

I think I may need a subquery, but I am not sure.

SELECT invoice_inventory.*, invoice_inventory_prices.* 
FROM invoice_inventory
LEFT JOIN invoice_inventory_prices 
   ON  `invoice_inventory`.`inventory_id`
      = `invoice_inventory_prices`.`inventory_id`
WHERE (company_id = 2 OR company_id = 0)
GROUP BY invoice_inventory.inventory_id 
ORDER BY company_id DESC

Answer 1

I am going to use two LEFT JOINs the first will go get the Default prices dp alias and the second will get the Special prices (company_id=2) sp alias

The CASE is used to determine if a Special price exists, use it, otherwise use the default.

You did not give a column list, so I took a guess at the name of the price column. If you require a second (or more) column, you can add additional CASE statements.

SELECT i.inventory_id
   , CASE
        WHEN sp.Price IS NOT NULL THEN sp.Price
        ELSE dp.Price
     END AS PriceForItem
FROM invoice_inventory AS i
LEFT JOIN invoice_inventory_prices AS dp ON  i.inventory_id = dp.inventory_id
   AND dp.company_id = 0
LEFT JOIN invoice_inventory_prices AS sp ON  i.inventory_id = sp.inventory_id
   AND sp.company_id = 2
GROUP BY invoice_inventory.inventory_id