IF Statement

The IF statement lets you execute a sequence of statements conditionally. Whether the sequence is executed or not depends on the value of a Boolean expression. For more information, see "Conditional Control: IF and CASE Statements".

Syntax

Text description of the illustration if_statement.gif

Keyword and Parameter Description

boolean_expression

This is an expression that yields the Boolean value TRUE, FALSE, or NULL. It is associated with a sequence of statements, which is executed only if the expression yields TRUE.

ELSE

If control reaches this keyword, the sequence of statements that follows it is executed.

ELSIF

This keyword introduces a Boolean expression to be evaluated if the expression following IF and all the expressions following any preceding ELSIFs yield FALSE or NULL.

THEN

This keyword associates the Boolean expression that precedes it with the sequence of statements that follows it. If the expression yields TRUE, the associated sequence of statements is 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 yields TRUE. If the expression yields 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 yields 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 yields 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 yields TRUE, its associated sequence of statements is executed and control passes to the next statement. If all expressions yield 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 yield TRUE. Nevertheless, order_quantity is assigned the proper value of 50 because processing of an IF statement stops after an expression yields 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

Expressions