It is not working when use trigger to check data from other table.

Question

Please help me with this, I have put a trigger on a table but it can not work as I expect.
case study: one class has many students, only one of them goes to match.
The purpose of this trigger is to check when choose a student goes to match, this student must in his class where he belongs to.
Version of Oracle is 10.2.0.1.0.
--table:
DROP TABLE STU;
DROP TABLE CLASS;
create table CLASS(
CID     VARCHAR2(5)   PRIMARY KEY,
CNAME   VARCHAR2(20)  NOT NULL,
SCHOSEN VARCHAR2(5));
create table STU(
SID     VARCHAR2(5)   PRIMARY KEY,
SNAME   VARCHAR2(20)  NOT NULL,
CID     VARCHAR2(5)   NOT NULL REFERENCES CLASS(CID) ON DELETE CASCADE);
--data:
--CLASS
INSERT INTO CLASS(CID,CNAME) VALUES(1,'SUN');
INSERT INTO CLASS(CID,CNAME) VALUES(2,'MOON');
INSERT INTO CLASS(CID,CNAME) VALUES(3,'EARTH');
--STU
INSERT INTO STU VALUES(1,'JACK',1);
INSERT INTO STU VALUES(2,'TOM',1);
INSERT INTO STU VALUES(3,'LILY',2);
INSERT INTO STU VALUES(4,'DUSTIN',3);
--TRIGGER
CREATE OR REPLACE TRIGGER CHECK_SCHOSEN
BEFORE INSERT OR UPDATE OF SCHOSEN ON CLASS
FOR EACH ROW WHEN (NEW.SCHOSEN IS NOT NULL)
DECLARE
DUMMY INTEGER;
INVALID_STU EXCEPTION;
VALID_STU EXCEPTION;
MUTATING_TABLE EXCEPTION;
PRAGMA EXCEPTION_INIT(MUTATING_TABLE, -4091);
CURSOR DUMMY_CURSOR (ST VARCHAR2, CL VARCHAR2) IS
  SELECT SID FROM STU, CLASS
  WHERE STU.SID=ST AND STU.CID=CLASS.CID AND CLASS.CID=CL
    FOR UPDATE OF CLASS.SCHOSEN;
BEGIN
  OPEN DUMMY_CURSOR(:NEW.SCHOSEN, :NEW.CID);
  FETCH DUMMY_CURSOR INTO DUMMY;
  IF DUMMY_CURSOR%NOTFOUND THEN
    RAISE INVALID_STU;
  ELSE
    RAISE VALID_STU;
  END IF;
  CLOSE DUMMY_CURSOR;
EXCEPTION
  WHEN INVALID_STU THEN
    CLOSE DUMMY_CURSOR;
    DBMS_OUTPUT.PUT_LINE('PLEASE RE-ENTER CLASS ID AND STUDENT ID AS CLASS OR STUDENT IS NOT VALID.');
  WHEN VALID_STU THEN
    CLOSE DUMMY_CURSOR;
    DBMS_OUTPUT.PUT_LINE('STUDENT CHOOSE SUCCEFULLY!');
  WHEN MUTATING_TABLE THEN
    NULL;
END;
/Just copy and paste above and try to run following:
UPDATE CLASS
SET SCHOSEN=3
WHERE CID=1;
Clearly,you can not choose student which is 3 as match member of class 1. Please help me. Thanks.
Edited by: 991096 on 01-Mar-2013 02:36
Edited by: 991096 on 01-Mar-2013 03:03
Edited by: 991096 on 01-Mar-2013 03:11

Answer 1

Hi,
Welcome to the forum!
Like the others, I can't see a business purpose for this trigger. I assume it's just an experiment to learn how triggers work.
991096 wrote:
Please help me with this, I have put a trigger on a table but it can not be triggered.Sorry, I don't understand what you want help with.
Are you saying the trigger doesn't fire? It does fire; it just raises the mutating table error, so you don't see anything.
case study: one class has many students, only one of them goes to match.What do you mean when you say "goes to match"?
--table:
DROP TABLE STU;
DROP TABLE CLASS;
create table CLASS(
CID VARCHAR2(5) PRIMARY KEY,
CNAME VARCHAR2(20) NOT NULL,
SCHOSEN VARCHAR2(5));
create table STU(
SID VARCHAR2(5) PRIMARY KEY,
SNAME VARCHAR2(20) NOT NULL,
CID VARCHAR2(5) NOT NULL REFERENCES CLASS(CID) ON DELETE CASCADE);
--data:
--CLASS
INSERT INTO CLASS(CID,CNAME) VALUES(1,'SUN');
INSERT INTO CLASS(CID,CNAME) VALUES(2,'MOON');
INSERT INTO CLASS(CID,CNAME) VALUES(3,'EARTH');
--STU
INSERT INTO STU VALUES(1,'JACK',1);
INSERT INTO STU VALUES(2,'TOM',1);
INSERT INTO STU VALUES(3,'LILY',2);
INSERT INTO STU VALUES(4,'DUSTIN',3);
--TRIGGER
CREATE OR REPLACE TRIGGER CHECK_SCHOSEN
BEFORE INSERT OR UPDATE OF SCHOSEN ON CLASS
FOR EACH ROW WHEN (NEW.SCHOSEN IS NOT NULL)
DECLARE
DUMMY INTEGER;
INVALID_STU EXCEPTION;
VALID_STU EXCEPTION;
MUTATING_TABLE EXCEPTION;
PRAGMA EXCEPTION_INIT(MUTATING_TABLE, -4091);
CURSOR DUMMY_CURSOR (ST VARCHAR2, CL VARCHAR2) IS
SELECT SID FROM STU, CLASS
WHERE STU.SID=ST AND STU.CID=CLASS.CID AND CLASS.CID=CL
FOR UPDATE OF CLASS.SCHOSEN;
BEGIN
OPEN DUMMY_CURSOR(:NEW.SCHOSEN, :NEW.CID);
FETCH DUMMY_CURSOR INTO DUMMY;
IF DUMMY_CURSOR%NOTFOUND THEN
RAISE INVALID_STU;
ELSE
RAISE VALID_STU;
END IF;
CLOSE DUMMY_CURSOR;
EXCEPTION
WHEN INVALID_STU THEN
CLOSE DUMMY_CURSOR;
DBMS_OUTPUT.PUT_LINE('PLEASE RE-ENTER CLASS ID AND STUDENT ID AS CLASS OR STUDENT IS NOT VALID.');
WHEN VALID_STU THEN
CLOSE DUMMY_CURSOR;
DBMS_OUTPUT.PUT_LINE('STUDENT CHOOSE SUCCEFULLY!');
WHEN MUTATING_TABLE THEN
NULL;
END;
/Thanks for posting the CREATE TABLE, INSERT and CREATE TRIGGER statements; that really helps!
See the forum FAQ {message:id=9360002} for other helpful tips, such as how to use \ tags to post formatted code.
Just copy and paste above and try to run following:
UPDATE CLASS
SET SCHOSEN=3
WHERE CID=1;
Clearly,you can not choose student which is 3 as match member of class 1. The trigger did not be triggered. Please help me. Thanks.You can't do DML, or even query, the class table from a FOR EACH ROW trigger on the same class table.
Add 3 more calls to put_line to see this:CREATE OR REPLACE TRIGGER CHECK_SCHOSEN
BEFORE INSERT OR UPDATE OF SCHOSEN ON CLASS
FOR EACH ROW WHEN (NEW.SCHOSEN IS NOT NULL)
DECLARE
DUMMY          INTEGER;
INVALID_STU      EXCEPTION;
VALID_STU          EXCEPTION;
MUTATING_TABLE      EXCEPTION;
PRAGMA           EXCEPTION_INIT (MUTATING_TABLE, -4091);
CURSOR DUMMY_CURSOR (ST VARCHAR2, CL VARCHAR2) IS
SELECT SID
     FROM      STU
     ,     CLASS
     WHERE     STU.SID          = ST
     AND     STU.CID          = CLASS.CID
     AND     CLASS.CID     = CL
     FOR UPDATE OF           CLASS.SCHOSEN;
BEGIN
dbms_output.put_line (:NEW.schosen || ' = schosen entering check_schosen');     -- ***** NEW *****
OPEN DUMMY_CURSOR (:NEW.SCHOSEN, :NEW.CID);
dbms_output.put_line ('Cursor is open now.');     -- ***** NEW *****
FETCH DUMMY_CURSOR INTO DUMMY;
IF DUMMY_CURSOR%NOTFOUND THEN
RAISE INVALID_STU;
ELSE
RAISE VALID_STU;
END IF;
CLOSE DUMMY_CURSOR;
EXCEPTION
WHEN INVALID_STU THEN
CLOSE DUMMY_CURSOR;
     DBMS_OUTPUT.PUT_LINE ('PLEASE RE-ENTER CLASS ID AND STUDENT ID AS CLASS OR STUDENT IS NOT VALID.');
WHEN VALID_STU THEN
     CLOSE DUMMY_CURSOR;
     DBMS_OUTPUT.PUT_LINE ('STUDENT CHOOSE SUCCEFULLY!');
WHEN MUTATING_TABLE THEN
DBMS_OUTPUT.PUT_LINE ('MUTATING TABLE');     -- ***** NEW *****
     NULL;
END;
Output, when trying to UPDATE class:3 = schosen entering check_schosen
MUTATING TABLE
1 row updated.
Obviously, the trigger fired, since you see the message "3 = schosen entering check_schosen"
Obviously, the OPEN statement raised an error, since you don't see the message 'Cursor is open now.'
Obviously, the mutating table error was raised, since you see the message 'MUTATING TABLE'.
So the trigger fired, and went to the EXCEPTION handler almost immediately.  The EXCEPTION handler printed a message, as instructed, and the trigger ended.  Then the actual UPDATE took place.
I hope this answers your question.
If not, what is your question?  It may be very clear to you, but a complete mystery to others.  Ask clear questions, such as "Why did ... happen?  I thought ... would happen, be ... as the PL/SQL manual says at ...".