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Predicates I page 1

Predicate is an expression taking truth value. It may be both a single expression or any combination of a numberless expressions built by means of Boolean operators AND, OR, and NOT. Besides, these combinations may inclide SQL-operator IS, and parentheses that define the order in which operations are to be executed.

 SQL(Structured Query Language) is a database computer language designed for the retrieval and management of data in relational database management systems (RDBMS), database schema creation and modification, and database object access control management.SQL predicate evaluates to TRUE, FALSE and UNKNOWN. The following predicates are exceptions to this rule: IS NULL, EXISTS, UNIQUE, and MATCH, which could not evaluate to UNKNOWN.

To remember combination rules for these three truth values, let us denote TRUE as 1, FALSE as 0, and UNKNOWN as 1/2 (somewhere between true and false) [2].

  •   AND with two truth values gives minimum of these values. For example, TRUE AND UNKNOWN is UNKNOWN.
  •   OR with two truth values gives maximum of these values. For example, FALSE OR UNKNOWN is UNKNOWN.
  •   Negation of truth value is 1 minus this truth value. For example, NOT UNKNOWN is UNKNOWN

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