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Collections are a way for you to organize kata so that you can create your own training routines. Every collection you create is public and automatically sharable with other warriors. After you have added a few kata to a collection you and others can train on the kata contained within the collection.
Get started now by creating a new collection.
That is incorrect. You might be misunderstanding what the group by is doing in the query. Your scenario exists in the DVD Rental sample database and the test case passes on the query.