#1242 - Subquery retourneert meer dan 1 rij - mysql

Aangezien uw subquery meerdere waarden kan retourneren, IN zou in de youy where-clausule moeten passen.

SELECT pictures_archive_filename 
FROM pictures_archive 
WHERE pictures_archive_id IN 
(
   SELECT pictures_archive_id 
   FROM pictures_archive_category 
   WHERE pictures_category_id = 9
)

een alternatief hiervoor is om join beide tabellen wat efficiënter is.

SELECT  pictures_archive_filename 
FROM    pictures_archive a 
        INNER JOIN pictures_archive_category b
            ON a.pictures_archive_id = b.pictures_archive_id
WHERE   b.pictures_category_id = 9