Don't use DISTINCT as a "join-fixer"
This is such a pervasive issue with the relational SQL query model ! The fact that joins always create brand new tables is powerful, but it's rarely what is needed.The example in this article can manage to do without a join because it's only using the other tables to filter the customer table. The proposed solution breaks down as soon as a column needs to be brought in from the other tables (for example, "the total number of products purchased from the Beauty category"). A notion as simple as "I want to return a list of customers with additional properties" does not map to any dedicated SQL construct ! You can JOIN then GROUP BY, but making sure that the result only has exactly one value per customer is not something you can explicitly say in your SQL, it's a consequence of how you set up your JOIN and GROUP BY keys, and it's both easy to make a mistake and hard to understand the intent.