Question

I have a problem with tables joining.

I have two tables: product_commons and product_prices. Product_prices has records with standard product prices + pre-generated prices for campaigns (campaign_id). I need a query which returns me list of products with campaign prices (if exists) or standard prices (if they not).

Something like this:

SELECT * FROM product_commons 
   INNER JOIN product_prices ON product_commons.id = product_prices.product_id 
WHERE (campaign_id = 3 OR campaign_id IS NULL) 
GROUP BY product_commons.id

Unfortunnely this query returns me prices only with campaign_id = NULL.

Answer 1

You need to use a mix of INNER and LEFT (OUTER) JOINs with a COALESCE, with the campaign_id filter in the JOIN conditions. To get 2 prices from different rows into one row, you need 2 JOINs.

SELECT
   *,
   COALESEC(pstd.price, pc.price)
FROM 
   product_commons pc
   JOIN
   product_prices pstd ON pc.id = pstd.product_id AND pstd.campaign_id IS NULL;
   LEFT JOIN 
   product_prices pp ON pc.id = pp.product_id AND pp.campaign_id = 3;

You don’t need a GROUP BY either: it doesn’t make sense (and would give errors used like this) in standard SQL or other RDBMS