Build an existence check for "one leading id, matched against a list of ids". The returned checker binds leadingId to the first ? and expands ids into the IN (...) your buildSql embeds via the placeholder string it receives. Shared by the per-attendee "across these listings" probes so their signature and args boilerplate live in one place. Empty ids still runs the query with an empty IN (), which matches nothing — callers pass a non-empty list.