When Correct SQL Returns the Wrong Answer
A query can run cleanly and still answer a different question. See how joins, NULLs, ordering, and aggregation levels change results, and how to catch it.
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Anti-Join
An anti-join finds rows with no match on the other side — customers who never ordered. One NULL can make NOT IN return nothing; NOT EXISTS does not.
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SQL from Zero to Working: Filters, Joins, Windows, and Transactions
You will write more SQL than any other language in this field. Here is the part that matters.