The use of a doubly nested set of NOT EXISTS SELECT statements is so rare that even if you are a professional database developer you will probably never see it used.

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Doubly nested NOT EXISTS SELECT statements, while not common in everyday SQL queries, are used in more complex database operations to solve specific problems, such as checking for the absence of certain data patterns across multiple tables. Their rarity does not mean a professional database developer will never encounter or need them.

The Use of a Doubly Nested Set of NOT EXISTS