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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.
No, you don't have to use join on a particular column. In fact, most of the solutions on here are what I would call bad practices in that they actually require data from the toys table to complete. You should really just do a count(1) so that when the join is happening underneath it could just use the index to get to the answer instead of requiring a table lookup for the value.