SQL ALTER TABLE ON DELETE CASCADE

Question

I have the following tables:

CREATE TABLE BOOK_AUTHORS
(Book_id CHAR(20) NOT NULL,
AuthorName VARCHAR(30) NOT NULL,
PRIMARY KEY (Book_id, AuthorName),
FOREIGN KEY (Book_id) REFERENCES BOOK (Book_id));

CREATE TABLE BOOK_COPIES
(Book_id CHAR(20) NOT NULL,
Branch_id CHAR(20) NOT NULL,
No_of_copies NUMBER,
PRIMARY KEY (Book_id, Branch_id),
FOREIGN KEY (Book_id) REFERENCES BOOK (Book_id),
FOREIGN KEY (Branch_id) REFERENCES LIBRARY_BRANCH (Branch_id));

I want to add ON DELETE CASCADE constraints to the both of them:

The first time I tried it said it worked. That file looks like:

ALTER TABLE "BOOK_AUTHORS"
ADD CONSTRAINT "fk_test"
FOREIGN KEY ("Book_id")
REFERENCES "BOOK" ("Book_id")
ON DELETE CASCADE;

Then I went through and made two separate tables for the two foreign keys in the second table:

ALTER TABLE "BOOK_COPIES"
ADD CONSTRAINT "fk_test1"
FOREIGN KEY ("Book_id")
REFERENCES "BOOK" ("Book_id")
ON DELETE CASCADE;

ALTER TABLE "BOOK_COPIES"
ADD CONSTRAINT "fk_test2"
FOREIGN KEY ("Branch_id")
REFERENCES "LIBRARY_BRANCH" ("Branch_id")
ON DELETE CASCADE;

However, upon doing so I got the errors

"Book_id" invalid identifier

and then

"Branch_id" invalid identifier

I don't know what I did wrong. I then went back and did the first alter table again (the one that I originally thought worked) and it gave me the same error message ( "Book_id" invalid identifier ). Can someone help me add these constraints? I also have five other tables to add these constraints to.

Answer 1

If you put double quotes around your identifiers (like you did in

ALTER TABLE "BOOK_COPIES"
ADD CONSTRAINT "fk_test1"
FOREIGN KEY ("Book_id")
REFERENCES "BOOK" ("Book_id")
ON DELETE CASCADE;

) your identifiers (eg "Book_id" in this case) become case-sensitive.

So either you'll have to change your table definition and rename the column to "Book_id" or (much preferably IMHO) just get rid of the double quotes in your constraint definition:

ALTER TABLE BOOK_COPIES
ADD CONSTRAINT fk_test1
FOREIGN KEY (Book_id)
REFERENCES BOOK (Book_id)
ON DELETE CASCADE;

Answer 2

First of all let me clear one thing, You cant add on delete cascade to an already existing foreign key constraint, as shown in docs you can only change its state which means enable or disable, in case if you need to add then drop the constraint first. This question is asked twice and still repeating please moderators have a glance on this. Here are links that has already solved your problem. first , second and this third and who knows how many questioned asked on on delete cascade.

Answer 3

Put doublequotes (") around your table and column names.

I added table "BOOK" and table "LIBRARY_BRANCH":

CREATE TABLE "BOOK"
("Book_id" CHAR(20) NOT NULL,
"BookName" VARCHAR(30) NOT NULL,
PRIMARY KEY ("Book_id"));

CREATE TABLE "BOOK_AUTHORS"
("Book_id" CHAR(20) NOT NULL,
"AuthorName" VARCHAR(30) NOT NULL,
PRIMARY KEY ("Book_id", "AuthorName"));

CREATE TABLE "LIBRARY_BRANCH" 
("Branch_id" CHAR(20) NOT NULL, 
 "Branch_name" VARCHAR(50), 
 "Address" VARCHAR(100), 
 PRIMARY KEY ("Branch_id"));

CREATE TABLE "BOOK_COPIES"
("Book_id" CHAR(20) NOT NULL,
"Branch_id" CHAR(20) NOT NULL,
"No_of_copies" NUMBER,
PRIMARY KEY ("Book_id", "Branch_id"));


ALTER TABLE "BOOK_AUTHORS"
ADD CONSTRAINT "fk_test"
FOREIGN KEY ("Book_id")
REFERENCES "BOOK" ("Book_id")
ON DELETE CASCADE;

ALTER TABLE "BOOK_COPIES"
ADD CONSTRAINT "fk_test1"
FOREIGN KEY ("Book_id")
REFERENCES "BOOK" ("Book_id")
ON DELETE CASCADE;

ALTER TABLE "BOOK_COPIES"
ADD CONSTRAINT "fk_test2"
FOREIGN KEY ("Branch_id")
REFERENCES "LIBRARY_BRANCH" ("Branch_id")
ON DELETE CASCADE;

sqlfiddle demo