Keyword and Parameter Description:

boolean_expression:

This is an expression which results the Boolean value TRUE, FALSE, & NULL. It is related with a series of statements that is executed only if the expression results TRUE.

THEN

This keyword relates the Boolean expression which precedes it with the series of statements that follows it. When the expression yields TRUE, the related series of statements is executed.

ELSIF:

This keyword introduce a Boolean expression to be computed if the expression following IF and all the expressions following any preceding ELSIFs results FALSE or NULL.

ELSE:

If control reaches this keyword, the series of statements which follows it is executed.