I use "ON DELETE CASCADE" regularly but I never use "ON UPDATE CASCADE" as I am not so sure in what situation it will be useful.
For the sake of discussion let see some code.
CREATE TABLE parent (
id INT NOT NULL AUTO_INCREMENT,
PRIMARY KEY (id)
CREATE TABLE child (
id INT NOT NULL AUTO_INCREMENT, parent_id INT,
INDEX par_ind (parent_id),
FOREIGN KEY (parent_id)
ON DELETE CASCADE
For "ON DELETE CASCADE", if a parent with an id is deleted, a record in a child with parent_id = parent.id will be automatically deleted. This should be no problem.
Does this mean that "ON UPDATE CASCADE" will do the same thing when id of the parent is updated?
If (1) is true, it means that there is no need to use "ON UPDATE CASCADE" if parent.id is not updatable (or will never be updated) like when it is AUTO_INCREMENT or always set to be TIMESTAMP. Is that right?
If (2) is not true, in what other kinds of situations should we use "ON UPDATE CASCADE"?
What if I (for some reason) update the child.parent_id to be something not existing, will it then be automatically deleted?
Well, I know, some of the questions above can be tested programmatically to understand but I want also to know if any of this is database vendor dependent or not.
Please shed some light.