Newbie needs some simple help on triggers

Question

Hi there. I need to create a trigger and am very new to this language and would love some help.
I have tables STUDENT, BOOKS, BORROWING.
When a student borrows a book, a composite entry is created in the BORROWING table. In the Books table, their is an attribute 'Borrowed' with a Y or N.
When a new record with the same Title as a book is created in borrowing, the value of 'borrowed' needs to be set to Y. So far I have the following, but forgot that I need to insert a WHERE clause so that it doesnt update every book, but only the book that causes the trigger.
I am having trouble with this- how would i go about doing this? Any help appreacited...
CREATE OR REPLACE TRIGGER book_returned_trigger
AFTER DELETE ON borrowing
FOR EACH ROW
BEGIN
UPDATE books
SET
borrowed= 'N';
END;

Answer 1

Hi guys. Am really having some trouble now. Funny thing is I had it working!
I have the following tables:
>
CREATE TABLE STUDENT
     studentID VARCHAR2(8) NOT NULL,
     firstname VARCHAR2(20) NOT NULL,
     lastname VARCHAR2(20) NOT NULL,
     gender VARCHAR2(10),
     dob DATE,
     address VARCHAR2(20),
     suburb VARCHAR2(20),
     phone VARCHAR2(8),
     PRIMARY KEY(studentID)
CREATE TABLE BOOKS
     title VARCHAR2(20) NOT NULL,
     publisher VARCHAR2(20),
     year DATE,
     borrowed CHAR(1) CHECK (borrowed IN ('Y','N')),
     PRIMARY KEY(title)
CREATE TABLE BORROWING
     studentID VARCHAR2(8) NOT NULL,
     title VARCHAR2(20) NOT NULL,
     PRIMARY KEY(studentID,title),
     FOREIGN KEY(studentID) REFERENCES student(studentID),
     FOREIGN KEY(title) REFERENCES books(title)
>
My Triggers look like this:
>
CREATE OR REPLACE TRIGGER book_borrowed_trigger
AFTER INSERT ON borrowing
FOR EACH ROW
BEGIN
UPDATE books
SET
borrowed= 'Y' WHERE title = :OLD.title;
END;
CREATE OR REPLACE TRIGGER book_returned_trigger
AFTER DELETE ON borrowing
FOR EACH ROW
BEGIN
UPDATE books
SET
borrowed= 'N' WHERE title = :OLD.title;
END;
>
and some sample date:
>
INSERT INTO student (studentID, firstname,
lastname, gender, dob, address, suburb, phone)
VALUES ('34025492','Sally','Field','Female',
'26/FEB/1970','123 Latrobe Pde','Dromana','59812553');
INSERT INTO student (studentID, firstname,
lastname, gender, dob, address, suburb, phone)
VALUES ('98503345','Ray','Blackburn','Female',
'9/NOV/1954','52 Murawa Dve','Sorrento','59812433');
INSERT INTO books (title, publisher, year, borrowed)
VALUES ('Harry Potter','Roundhouse','1998','N');
INSERT INTO books (title, publisher, year, borrowed)
VALUES ('Twilight','Jones Books','2000','N');
INSERT INTO borrowing (studentID, title)
VALUES ('20737785','Harry Potter');
>
Now I cant get this trigger to change the value of the 'borrowed' column in 'BOOKS' to Y when I create a new record in the BORROWING table that references one of the Books.
Any idea why? I cant see why! Thanks for the help so far guys
Edited by: lockmac on Apr 1, 2009 12:40 AM