IF 语句根据一个布尔表达式的值决定是执行还是跳过一序列的语句。语法格式如下：

    1. IF boolean_expression THEN statement [, statement ]
    2. [ ELSIF boolean_expression THEN statement [, statement ] ]...
    3. [ ELSE statement [, statement ] ]
    4. END IF;

    示例：使用 IF 语句

    1. delimiter /
    2. CREATE OR REPLACE PROCEDURE sp_guess(p_value IN number)
    3. IS
    4. BEGIN
    5.     IF p_value IS NULL THEN
    6.         dbms_output.put_line('The value is NULL!');
    7.     ELSIF (p_value < 0) THEN
    8.         dbms_output.put_line('The value is a negative number. ');
    9.     ELSE 
    10.         dbms_output.put_line('The value is a positive number. ');   
    11.     END IF; 
    12. END;
    13. /
    15. delimiter ;
    17. obclient>
    18. obclient> set serveroutput on;
    19. Query OK, 0 rows affected (0.00 sec)
    21. obclient> call sp_guess(2020);
    22. Query OK, 0 rows affected (0.03 sec)
    24. The value is a positive number.
    25. obclient> call sp_guess(-2000);
    26. Query OK, 0 rows affected (0.00 sec)
    28. The value is a negative number.
    29. obclient>