根据左联接返回不同的记录

Question

I'm having some trouble formulating a complex SQL query. I'm getting the result I'm looking for and the performance is fine but whenever I try to grab distinct rows for my LEFT JOIN of product_groups, I'm either hitting some performance issues or getting incorrect results.

Here's my query:

 SELECT
    pl.name, pl.description,
    p.rows, p.columns,
    pr.sku,
    m.filename, m.ext, m.type,
    ptg.product_group_id AS group,
 FROM
    product_region AS pr
 INNER JOIN
    products AS p ON (p.product_id = pr.product_id)
 INNER JOIN
    media AS m ON (p.media = m.media_id)
 INNER JOIN
    product_language AS pl ON (p.product_id = pl.product_id)
 LEFT JOIN
    products_groups AS ptg ON (ptg.product_id = pr.product_id)
 WHERE
    (pl.lang = :lang) AND
    (pr.region = :region) AND
    (pt.product_id = p.product_id)
 GROUP BY
    p.product_id
 LIMIT
    :offset, :limit

The result I'm being given is correct however I want only distinct rows returned for "group". For example I'm getting many results back that have the same "group" value but I only want the first row to show and the following records that have the same group value to be left out.

I tried GROUP BY group and DISTINCT but it gives me incorrect results. Also note that group can come back as NULL and I don't want those rows to be effected.

Thanks in advance.

Answer 1

I worked out a solution an hour after posting this. My goal was to group by product_group_id first and then the individual product_id. The requirement was that I would eliminate product duplicates and have ONE product represent the group set.

I ended up using COALESCE(ptg.product_group_id, p.product_id). This accounts for the fact that most of my group IDs were null except for a few dispersed products. In using COALESCE I'm first grouping by the group ID, if that value is null it ignores the group and collects by product_id.