GROUP or DISTINCT after JOIN returns duplicates

Question

I have two tables, products and meta. They are in relation 1:N where each product row has at least one meta row via foreign key.

(viz. SQLf

Answer 1

You can use a subquery to identify the max(ID) for each product, then use that in the superquery to gather the details you want to display:

SELECT q.product_id, meta.* from
(SELECT product_id, max(meta.ID)
 FROM meta JOIN products ON products.id=meta.product_id 
 GROUP BY product_id) q 
JOIN meta ON q.max=meta.id;

It is not the only solution!

A quick comparison to use of DISTINCT ON solutions suggests that it is slower (http://sqlfiddle.com/#!15/c8f34/38). It avoids a full sort on ID and prefers a sequential scan.