Does designating a foreign key automatically take care of updates to the foreign table's value? - sql

Question

I have two tables: student and faculty
I added a column to the student table that is a foreign key that references a column in faculty.
Let's say that a faculty's id changes. Based on my code below, will my student table update accordingly? Or do I need to do anything extra to ensure that it updates? For example, pretend that James's id updates from 1 to 99. Will the advisorid column of student update accordingly?
Note - I am using LINQPad and I have no way to test this because when I try to make a change to the referenced table I get an Error 547: The UPDATE statement conflicted with the REFERENCE constraint "FK__enroll__studenti__178D7CA5". The conflict occurred in database "tempdb", table "dbo.enroll", column 'studentid'.
alter table student
add advisorid int foreign key(advisorid) references faculty(facultyid);
update student set advisorid = 1 where studentid = 1;
select * from student
select * from faculty;

Answer 1

As you've seen, by default, updates on the key a foreign key is based of do no propagate to the dependent columns. You can get such a behavior by specifying an on update clause:
alter table student
add advisorid int foreign key(advisorid) references faculty(facultyid)
on update cascade;
EDIT:
To answer the question in the comments, the behavior could also be defined when deleting a row:
alter table student
add advisorid int foreign key(advisorid) references faculty(facultyid)
on delete cascade
on update cascade;

Answer 2

In Oracle ON UPDATE CASCADE does not work. Here I will demonstrate an alternative update scenario.
Instead of specifying ON UPDATE CASCADE it is possible to deferring the check of a contraint to the end of a transaction. Therefore the constraint must be DEFERRABLE. Unfortuantely the constraint could not be modified so that it must dropped and recreated.
Here is an Example:
CREATE TABLE parent_ (
pid NUMBER CONSTRAINT pk_parent PRIMARY KEY,
c1 VARCHAR2(50)
);
CREATE TABLE child_ (
cid NUMBER,
pid NUMBER CONSTRAINT fk_child_parent REFERENCES parent_(pid) ON DELETE CASCADE, --DEFERRABLE,
c1 VARCHAR2(50)
);
INSERT INTO parent_ VALUES (1, 'two');
INSERT INTO child_ VALUES (2, 1, 'two');
COMMIT;
An update of the primary key or foreign key will lead to an oracle exception
UPDATE parent_ SET pid = 2
Fehlerbericht -
SQL-Fehler: ORA-02292: integrity constraint (ME.FK_CHILD_PARENT) violated - child record found
02292. 00000 - "integrity constraint (%s.%s) violated - child record found"
*Cause: attempted to delete a parent key value that had a foreign
dependency.
*Action: delete dependencies first then parent or disable constraint.
UPDATE child_ SET pid = 2
Fehlerbericht -
SQL-Fehler: ORA-02291: integrity constraint (ME.FK_CHILD_PARENT) violated - parent key not found
02291. 00000 - "integrity constraint (%s.%s) violated - parent key not found"
*Cause: A foreign key value has no matching primary key value.
*Action: Delete the foreign key or add a matching primary key.
Now the foreign key will be re-created as deferrable. It will still lead to foreign key constraint violations but it can be set to deferred per session. The advantage is that all other session still behave as before.
ALTER TABLE child_ DROP CONSTRAINT fk_child_parent;
ALTER TABLE child_ ADD CONSTRAINT fk_child_parent FOREIGN KEY (pid) REFERENCES parent_(pid) ON DELETE CASCADE DEFERRABLE;
ALTER SESSION SET CONSTRAINTS = DEFERRED;
UPDATE parent_ SET pid = 2;
UPDATE child_ SET pid = 2;
COMMIT;
ALTER SESSION SET CONSTRAINTS = IMMEDIATE;
The DEFERRABLE attribute can also be set when creating a foreign key (also inline). Find more on that in the official documentation