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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.
For people learnng SQL I think this Kata is a misleading example of HAVING. Sure, it gets the result, but grouping by id, which is a unique field?! 100% of developers would approach this problem using a where clause.