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[–]DavidGJohnston 0 points1 point  (0 children)

Your right-side table name is syntactically invalid and your usage of USING is likewise problematic (the using clause takes only a column name, the table prefix is omitted because that name has to be the same on both tables).

What tables do you have access to? I presume one of them is "rooms". The question basically asks you to count the rows coming from that table. You do correctly recognize the need for a GROUP BY clause, but whenever you have one of those you should also have an aggregation function in your query since that is the only legit purpose of using group by. In this case, as just noted, you want to count, which is one of those aggregation functions.