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 Data Types and SubtypesData Definition (DDL) Statements 

Predicates

A predicate is a simple expression asserting some fact, let’s call it P.If P resolves as TRUE, it succeeds.If it resolves to FALSE or NULL (UNKNOWN), it fails.A trap lies here, though: suppose the predicate, P, returns FALSE.In this case NOT(P) will return TRUE.On the other hand, if P returns NULL (unknown), then NOT(P) returns NULL as well.

In SQL, predicates can appear in CHECK constraints, WHERE and HAVING clauses, CASE expressions, the IIF() function and in the ON condition of JOIN clauses, and anywhere a normal expression can occur.

Conditions

A condition — or Boolean expression — is a statement about the data that, like a predicate, can resolve to TRUE, FALSE or NULL.Conditions consist of one or more predicates, possibly negated using NOT and connected by AND and OR operators.Parentheses may be used for grouping predicates and controlling evaluation order.

A predicate may embed other predicates.Evaluation sequence is in the outward direction, i.e. the innermost predicates are evaluated first.Each “level” is evaluated in precedence order until the truth value of the ultimate condition is resolved.