Another variant of SQL to answer your second question—reliable, fast and does not use subqueries at all :) 
SELECT i.* FROM items AS i 
  LEFT JOIN mapping AS m ON (m.item_id = i.item_id AND m.container_id=
+1) 
WHERE m.container_id IS NULL

[download]
'Fast' means that it is definitely not slower than than the variant with subqueries, and it can be better optimized by the server.

Generally, any EXCEPT statements can be rewritten using subqueries and often even using simple JOINs. Try this way :)

     s;;Just-me-not-h-Ni-m-P-Ni-lm-I-ar-O-Ni;;tr?IerONim-?HAcker ?d;print

In reply to Re: Combined SQL Queries (e.g., EXCEPT) and DBIx::Class by Ieronim
in thread Combined SQL Queries (e.g., EXCEPT) and DBIx::Class by jgallagher

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