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  • Morgan Helms thinks this is interesting:

SQL implements three-valued predicate logic by supporting the NULL mark to signify the generic concept of a missing value. Support for NULL marks and three-valued predicate logic in SQL is the source of a great deal of confusion and complexity, though one can argue that missing values are part of reality.

From

Cover of Microsoft® SQL Server® 2012 T-SQL Fundamentals

Note

SQL and its three-valued predicate logic