Boolean Data Type - SQL

SQL

The SQL:1999 standard introduced a BOOLEAN data type as an optional feature (T031). When restricted by a NOT NULL constraint, a SQL BOOLEAN behaves like Booleans in other languages. In SQL however, the BOOLEAN type is nullable by default like all other SQL data types, meaning it can have the special null value as well. Although the SQL standard defines three literals for the BOOLEAN type—TRUE, FALSE and UNKNOWN—, it also says that the NULL BOOLEAN and UNKNOWN "may be used interchangeably to mean exactly the same thing". This has caused some controversy because the identification subjects UNKNOWN to the equality comparison rules for NULL. More precisely UNKNOWN = UNKNOWN is not TRUE but UNKNOWN/NULL. As of 2012 few major SQL systems implement the T031 feature. PostgreSQL is a notable exception, although it does not implement the UNKNOWN literal; NULL can be used instead. (PostgreSQL does implement the IS UNKNOWN operator, which is part of an orthogonal feature, F571.) In other SQL implementations various ad-hoc solutions are used, like bit, byte, and character to simulate Boolean values.