Discussion
How can you find rows that do not match some specified condition?
Answer: B
Correct answer: Double use of NOT EXISTS
A single NOT EXISTS finds rows for which no matching row exists, which is not the same as rows failing to match a condition across a set. The classic relational division problem - find the rows that match every member of a set - is expressed as a doubly nested NOT EXISTS: there is no member of the set for which there is no matching row. It is awkward to read and is the standard idiom for the purpose.
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