E. F. Codd originally suggested two types of values: "unknown" and "missing". Somehow we got NULL, which represents both. ANSI SQL:1991 provides an optional fe

E. F. Codd originally suggested two types of values: "unknown" and "missing". Somehow we got NULL, which represents both. ANSI SQL:1991 provides an optional feature that introduces a special value UNKNOWN to boolean expressions [1]. But few databases support it. MSSQL [2] is one of the few that do. As I understand it, it's not a data type that be used in tables, but is only a potential result of boolean operations. [1] https://modern-sql.com/concept/three-valued-logic https://modern-sql.com/concept/three-valued-logic [2] https://learn.microsoft.com/en-us/sql/t-sql/language-elements/null-and-unknown-transact-sql https://learn.microsoft.com/en-us/sql/t-sql/language-element...