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".

Syntax

Description of the illustration if_statement.gif

Keyword and Parameter Description

boolean_expression

An expression that returns the Boolean value TRUE, FALSE, or NULL. Examples are comparisons for equality, greater-than, or less-than. The sequence following the THEN keyword is executed only if the expression returns TRUE.

ELSE

If control reaches this keyword, the sequence of statements that follows it is executed. This occurs when none of the previous conditional tests returned TRUE.

ELSIF

Introduces a Boolean expression that is evaluated if none of the preceding conditions returned TRUE.

THEN

If the expression returns TRUE, the statements after the THEN keyword are executed.

Usage Notes

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.

Examples

In the example below, if shoe_count has a value of 10, both the first and second Boolean expressions return TRUE. Nevertheless, order_quantity is assigned the proper value of 50 because processing of an IF statement stops after an expression returns TRUE and its associated sequence of statements is executed. The expression associated with ELSIF is never evaluated and control passes to the INSERT statement.

IF shoe_count < 20 THEN
   order_quantity := 50;
ELSIF shoe_count < 30 THEN
   order_quantity := 20;
ELSE
   order_quantity := 10;
END IF;

INSERT INTO purchase_order VALUES (shoe_type, order_quantity);

In the following example, depending on the value of score, one of two status messages is inserted into the grades table:

IF score < 70 THEN
   fail := fail + 1;
   INSERT INTO grades VALUES (student_id, 'Failed');
ELSE
   pass := pass + 1;
   INSERT INTO grades VALUES (student_id, 'Passed');
END IF;

Related Topics

CASE Statement, Expressions