Can't execute query

SELECT b.html, (0) as level FROM banners b, banners_categories bc WHERE bc.category_id=6 and b.bannerid=bc.banner_id UNION SELECT b.html, (1) as level FROM banners b, banners_categories bc WHERE bc.category_id=2 and b.bannerid=bc.banner_id UNION SELECT b.html, (2) as level FROM banners b, banners_categories bc WHERE bc.category_id=1 and b.bannerid=bc.banner_id ORDER BY level, RAND() limit 1

MySQL Error: Can't create/write to file '/var/tmp/#sql_db9_0.MYI' (Errcode: 28)

This script cannot continue, terminating.