limitmajors.sql

Oracle 9i PL/SQL程序设计的随书源码

REM LimitMajors.sql
REM Chapter 11, Oracle9i PL/SQL Programming by Scott Urman
REM This trigger will raise an ORA-4091 mutating table error.

CREATE OR REPLACE TRIGGER LimitMajors
  /* Limits the number of students in each major to 5.
     If this limit is exceeded, an error is raised through
     raise_application_error. */
  BEFORE INSERT OR UPDATE OF major ON students
  FOR EACH ROW
DECLARE
  v_MaxStudents CONSTANT NUMBER := 5;
  v_CurrentStudents NUMBER;
BEGIN
  -- Determine the current number of students in this
  -- major.
  SELECT COUNT(*)
    INTO v_CurrentStudents
    FROM students
    WHERE major = :new.major;

  -- If there isn't room, raise an error.
  IF v_CurrentStudents + 1 > v_MaxStudents THEN
    RAISE_APPLICATION_ERROR(-20000,
      'Too many students in major ' || :new.major);
  END IF;
END LimitMajors;
/

UPDATE students
  SET major = 'History'
  WHERE ID = 10003;

REM Drop the trigger, since it doesn't work.
DROP TRIGGER LimitMajors;