IF Statement
The IF statement executes or skips a sequence of statements, depending on the value of a Boolean expression. For more information, see Testing Conditions (IF and CASE Statements).
Description of the illustration if_statement.gif
Keyword and Parameter Descriptions
If and only if the value of this expression is TRUE, the statements following THEN execute.
If control reaches this keyword, the statements that follow it execute. This occurs when no boolean_expression had the value TRUE.
Introduces a boolean_expression that is evaluated if no preceding boolean_expression had the value TRUE.
If the expression returns TRUE, the statements after the THEN keyword are executed.
There are three forms of IF statements: IF-THEN, IF-THEN-ELSE, and IF-THEN-ELSIF. The simplest form of IF statement associates a Boolean expression with a sequence of statements enclosed by the keywords THEN and END IF. The sequence of statements is executed only if the expression returns TRUE. If the expression returns FALSE or NULL, the IF statement does nothing. In either case, control passes to the next statement.
The second form of IF statement adds the keyword ELSE followed by an alternative sequence of statements. The sequence of statements in the ELSE clause is executed only if the Boolean expression returns FALSE or NULL. Thus, the ELSE clause ensures that a sequence of statements is executed.
The third form of IF statement uses the keyword ELSIF to introduce additional Boolean expressions. If the first expression returns FALSE or NULL, the ELSIF clause evaluates another expression. An IF statement can have any number of ELSIF clauses; the final ELSE clause is optional. Boolean expressions are evaluated one by one from top to bottom. If any expression returns TRUE, its associated sequence of statements is executed and control passes to the next statement. If all expressions return FALSE or NULL, the sequence in the ELSE clause is executed.
An IF statement never executes more than one sequence of statements because processing is complete after any sequence of statements is executed. However, the THEN and ELSE clauses can include more IF statements. That is, IF statements can be nested.