Mysql function: validate Iran national code

check_national_code.sql

DELIMITER $$
CREATE FUNCTION CHECK_NATIONAL_CODE(national_code VARCHAR(50))
  RETURNS BOOLEAN
DETERMINISTIC
  BEGIN
    DECLARE c INT;
    DECLARE n INT;
    DECLARE r INT;
    
    IF length(national_code) != 10
    THEN
      RETURN FALSE;
    END IF;
    
    IF national_code REGEXP '^[0-9]'
    THEN
      
      IF (national_code = '1111111111' OR
          national_code = '0000000000' OR
          national_code = '2222222222' OR
          national_code = '3333333333' OR
          national_code = '4444444444' OR
          national_code = '5555555555' OR
          national_code = '6666666666' OR
          national_code = '7777777777' OR
          national_code = '8888888888' OR
          national_code = '9999999999')
      THEN
        RETURN FALSE;
      END IF;
      
      SET c = FLOOR(substr(national_code, 10, 1));
      SET n = FLOOR(substr(national_code, 1, 1)) * 10 +
              FLOOR(substr(national_code, 2, 1)) * 9 +
              FLOOR(substr(national_code, 3, 1)) * 8 +
              FLOOR(substr(national_code, 4, 1)) * 7 +
              FLOOR(substr(national_code, 5, 1)) * 6 +
              FLOOR(substr(national_code, 6, 1)) * 5 +
              FLOOR(substr(national_code, 7, 1)) * 4 +
              FLOOR(substr(national_code, 8, 1)) * 3 +
              FLOOR(substr(national_code, 9, 1)) * 2;
      SET r = n - (FLOOR((n / 11)) * 11);
      
      IF ((r = 0 AND r = c) OR (r = 1 AND c = 1) OR (r > 1 AND c = (11 - r)))
      THEN
        RETURN TRUE;
      ELSE
        RETURN FALSE;
      END IF;
      
    ELSE
      RETURN FALSE;
    END IF;
  END $$
DELIMITER ;