Conditional Statements

Conditional statements are used to decide whether given conditions are met. Operations are executed based on the decisions made.

GaussDB(DWS) supports five usages of IF:

IF_THEN

Figure 1 IF_THEN::=
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IF_THEN is the simplest form of IF. If the condition is true, statements are executed. If it is false, they are skipped.

Example

     
          IF v_user_id <> 0 THEN
    UPDATE users SET email = v_email WHERE user_id = v_user_id;
END IF;

IF_THEN_ELSE

Figure 2 IF_THEN_ELSE::=
Click to enlarge

IF-THEN-ELSE statements add ELSE branches and can be executed if the condition is false.

Example

     
          IF parentid IS NULL OR parentid = ''
THEN
    RETURN;
ELSE
    hp_true_filename(parentid); -- Call the stored procedure.
END IF;

IF_THEN_ELSE IF

IF statements can be nested in the following way:

     
          IF gender = 'm' THEN
    pretty_gender := 'man';
ELSE
    IF gender = 'f' THEN
        pretty_gender := 'woman';
    END IF;
END IF;

Actually, this is a way of an IF statement nesting in the ELSE part of another IF statement. Therefore, an END IF statement is required for each nesting IF statement and another END IF statement is required to end the parent IF-ELSE statement. To set multiple options, use the following form:

IF_THEN_ELSIF_ELSE

Figure 3 IF_THEN_ELSIF_ELSE::=
Click to enlarge

Example

     
          IF number_tmp = 0 THEN
    result := 'zero';
ELSIF number_tmp > 0 THEN 
    result := 'positive';
ELSIF number_tmp < 0 THEN
    result := 'negative';
ELSE
    result := 'NULL';
END IF;

IF_THEN_ELSEIF_ELSE

ELSEIF is an alias of ELSIF.

Example

      
           CREATE OR REPLACE PROCEDURE proc_control_structure(i in integer) 
AS
    BEGIN
        IF i > 0 THEN
            raise info 'i:% is greater than 0. ',i; 
        ELSIF i < 0 THEN
            raise info 'i:% is smaller than 0. ',i; 
        ELSE
            raise info 'i:% is equal to 0. ',i; 
        END IF;
        RETURN;
    END;
/

CALL proc_control_structure(3);

-- Delete the stored procedure:
DROP PROCEDURE proc_control_structure;