greatest-n-per-group.sql

SELECT c.*, p1.*
FROM customer c
JOIN purchase p1 ON (c.id = p1.customer_id)
LEFT OUTER JOIN purchase p2 ON (c.id = p2.customer_id AND 
    (p1.date < p2.date OR p1.date = p2.date AND p1.id < p2.id))
WHERE p2.id IS NULL;

Second example:

select pr.* from privacy_regulations as pr 
left outer join privacy_regulations as pr1 on (pr1.type = pr.type and pr1.version > pr.version)
where pr1.version is null
group by pr.type