Predicates are expressions that apply comparison operators (comp_op elements in the following syntax) and/or SQL predicate operators (such as IN and EXISTS) to values to produce a truth value of TRUE, FALSE, or UNKNOWN.
Predicates can be either a single expression or a combination of any number of expressions using Boolean operators (AND, OR, and NOT) as well as the special SQL operator IS, and parentheses to define the order of evaluation.
Predicates are most often used in the WHERE and HAVING clauses of SELECT statements and subqueries to determine the rows or aggregate groups to select, and in UPDATE and DELETE statements to identify the rows on which changes should be made.
Predicates evaluate to TRUE, FALSE, or UNKNOWN. UNKNOWNs arise when NULLs are compared to any value, including other NULLs, because it is impossible to know the value of a data field with NULL value. You can use Boolean operators and SQL IS on UNKNOWN truth values.