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The F571 feature is orthogonal to the presence of the [[boolean data type|boolean datatype]] in SQL (discussed later in this article) and, despite syntactic similarities, F571 does not introduce boolean or three-valued [[Literal (computer programming)|literals]] in the language. The F571 feature was actually present in [[SQL92]],<ref name="MeltonSimon1993">{{cite book|author1=Jim Melton|author2=Jim Melton Alan R. Simon|title=Understanding The New SQL: A Complete Guide|url=https://books.google.com/books?id=ZOOMSTZ4T_QC&pg=PA145|year=1993|publisher=Morgan Kaufmann|isbn=978-1-55860-245-8|pages=145–147}}</ref> well before the boolean datatype was introduced to the standard in 1999. The F571 feature is implemented by few systems however; PostgreSQL is one of those implementing it.
The addition of IS UNKNOWN to the other operators of SQL's three-valued logic makes the SQL three-valued logic [[functionally complete]],<ref>C. J. Date, ''Relational database writings, 1991-1994'', Addison-Wesley, 1995, p. 371</ref> meaning its logical operators can express (in combination) any conceivable three-valued logical function.
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