Conditional Statements

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

openGauss supports five usages of IF:

• IF_THEN

  Figure 1 IF_THEN::=
  

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

  Example:

  openGauss=# 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::=
  

  IF_THEN_ELSE statements add ELSE branches and can be executed if the condition is false.

  Example:

  openGauss=# 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:

  openGauss=# IF sex = 'm' THEN
      pretty_sex := 'man';
  ELSE
      IF sex = 'f' THEN
          pretty_sex := '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::=
  

  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;