I have two tables, a Photographer and an Influences table. In the Influences table, a Photographer is able to influence another Photographer.
Relational Schema:
Here is my Diagram in SQL:
Instance Example:
My issue is when I try to delete Photographer Tom, since he influences Jason (as shown in Influences table), I get a error stating:
The DELETE statement conflicted with the REFERENCE constraint "FK_Influences_Photographer1". The conflict occurred in database "jma59", table "dbo.Influences"
If Tom was in the column EPName of the Influences table, I have no issue deleting it. I know that this is a foreign key issue but I am not sure how to handle this situation. I created two separate foreign keys that reference to the Photographer primary key. But the problem is that I cannot make it so that both foreign keys cascade on update and delete.
First foreign key is EPName and EPBDate referencing to PName and PBDate of Photographer
Second foreign key is RPName and RPBDate referning to PName and PBDate of Photographer as well.
The error is:
Unable to create relationship 'FK_Influences_Photographer1'. Introducing FOREIGN KEY constraint 'FK_Influences_Photographer1' on table 'Influences' may cause cycles or multiple cascade paths. Specify ON DELETE NO ACTION or ON UPDATE NO ACTION, or modify other FOREIGN KEY constraints.
Any advice is greatly appreciated!
You could consider using a Trigger rather than a cascading delete to enforce referential integrity: https://support.microsoft.com/en-gb/help/321843/error-message-1785-occurs-when-you-create-a-foreign-key-constraint-tha
I have several tables which reference one main table using foreign keys.
My goal is for one blanket delete statement from the main table, which will then delete all those with the data as foreign keys as well.
However I get constraint errors as I expect since the reference is deleted before its deleted from the other tables. Is there any way to tell SQL I would like to delete from all tables where the data is used as a foreign key as well?
To have deletes on the parent table cascade to referencing tables, use an ON DELETE CASCADE foreign key. However, if you're deleting all rows from the main table there's a better way to solve your problem using TRUNCATE; see the end of this post.
You can ALTER TABLE to change a default ON DELETE NO ACTION foreign key to ON DELETE CASCADE. Drop the constraint then re-create it with the ON DELETE CASCADE modifier. See CREATE TABLE and the constraints documentation for information on the foreign key constraint syntax.
In brief, instead of col coltype REFERENCES fktable(fkcol) you use col coltype REFERENCES fktable(fkcol) ON DELETE CASCADE.
If you use ON DELETE CASCADE then to get even vaguely decent performance it is vital to create an index on the foreign key column. Even then, for bulk deletes from the target table it's often much faster to go and delete from each foreign key referencing table first, rather than relying on cascades.
You may want to also use ON UPDATE CASCADE if you're using ON DELETE CASCADE.
If you're deleting all rows from the main table, not just a selection of them, you're probably better off truncating the tables with TRUNCATE ... CASCADE:
CASCADE
Automatically truncate all tables that have foreign-key references to
any of the named tables, or to any tables added to the group due to
CASCADE.
The only way to have a DELETE cascade to tables with a foreign key linked to the table you are running your statement on is to have the ON DELETE CASCADE option set on the key, you will need to run an ALTER TABLE statement to change them from their current value to this option:
http://www.postgresql.org/docs/current/interactive/ddl-constraints.html
Our application uses an Oracle 10g database where several primary keys are exposed to the end user. Productcodes and such. Unfortunately it's to late to do anything with this, as there are tons of reports and custom scripts out there that we do not have control over. We can't redefine the primary keys or mess up the database structure.
Now some customer want to change some of the primary key values. What they initially wanted to call P23A1 should now be called CAT23MOD1 (not a real example, but you get my meaning.)
Is there an easy way to do this? I would prefer a script of some sort, that could be parametrized to fit other tables and keys, but external tools would be acceptable if no other way exists.
The problem is presumably with the foreign keys that reference the PK. You must define the foreign keys as "deferrable initially immediate", as described in this Tom Kyte article: http://www.oracle.com/technology/oramag/oracle/03-nov/o63asktom.html
That lets you ...
Defer the constraints
Modify the parent value
Modify the child values
Commit the change
Simple.
Oops. A little googling makes it appear that, inexplicably, Oracle does not implement ON UPDATE CASCADE, only ON DELETE CASCADE. To find workarounds google ORACLE ON UPDATE CASCADE. Here's a link on Creating A Cascade Update Set of Tables in Oracle.
Original answer:
If I understand correctly, you want to change the values of data in primary key columns, not the actual constraint names of the keys themselves.
If this is true it can most easily be accomplished redefining ALL the foreign keys that reference the affected primary key constraint as ON UPDATE CASCADE. This means that when you make a change to the primary key value, the engine will automatically update all related values in foreign key tables.
Be aware that if this results in a lot of changes it could be prohibitively expensive in a production system.
If you have to do this on a live system with no DDL changes to the tables involved, then I think your only option is to (for each value of the PK that needs to be changed):
Insert into the parent table a copy of the row with the PK value replaced
For each child table, update the FK value to the new PK value
Delete the parent table row with the old PK value
If you have a list of parent tables and the PK values to be renamed, it shouldn't be too hard to write a procedure that does this - the information in USER_CONSTRAINTS can be used to get the FK-related tables for a given parent table.
For instance, suppose I have table A. Then I have tables B-Z that have a foreign key to table A's primary key. Then perhaps there are also some tables that have a foreign key constraint to a table in B-Z's primary key constraint. Is there any easy way to clear out table A and all of the tables that refer to A (or that refer to a table that refers to A) without having to explicitly delete from each table or add an ON CASCADE constraint to each foreign key?
Note that this is mainly for testing purposes, not to be used in production. I would just drop the entire schema and start over again, but that simply isn't feasible for every test (considering how long it takes to build the schema).
I think the most efficient way to do this would be to drop all the FK's, truncate the tables, and then rebuild the FK's.
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)
REFERENCES parent(id)
ON DELETE CASCADE
);
For ON DELETE CASCADE, if a parent with an id is deleted, a record in child with parent_id = parent.id will be automatically deleted. This should be no problem.
This means 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 kind of situation 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 question above can be test programmatically to understand but I want also know if any of this is database vendor dependent or not.
Please shed some light.
It's true that if your primary key is just an identity value auto incremented, you would have no real use for ON UPDATE CASCADE.
However, let's say that your primary key is a 10 digit UPC bar code and because of expansion, you need to change it to a 13-digit UPC bar code. In that case, ON UPDATE CASCADE would allow you to change the primary key value and any tables that have foreign key references to the value will be changed accordingly.
In reference to #4, if you change the child ID to something that doesn't exist in the parent table (and you have referential integrity), you should get a foreign key error.
Yes, it means that for example if you do UPDATE parent SET id = 20 WHERE id = 10 all children parent_id's of 10 will also be updated to 20
If you don't update the field the foreign key refers to, this setting is not needed
Can't think of any other use.
You can't do that as the foreign key constraint would fail.
I think you've pretty much nailed the points!
If you follow database design best practices and your primary key is never updatable (which I think should always be the case anyway), then you never really need the ON UPDATE CASCADE clause.
Zed made a good point, that if you use a natural key (e.g. a regular field from your database table) as your primary key, then there might be certain situations where you need to update your primary keys. Another recent example would be the ISBN (International Standard Book Numbers) which changed from 10 to 13 digits+characters not too long ago.
This is not the case if you choose to use surrogate (e.g. artifically system-generated) keys as your primary key (which would be my preferred choice in all but the most rare occasions).
So in the end: if your primary key never changes, then you never need the ON UPDATE CASCADE clause.
Marc
A few days ago I've had an issue with triggers, and I've figured out that ON UPDATE CASCADE can be useful. Take a look at this example (PostgreSQL):
CREATE TABLE club
(
key SERIAL PRIMARY KEY,
name TEXT UNIQUE
);
CREATE TABLE band
(
key SERIAL PRIMARY KEY,
name TEXT UNIQUE
);
CREATE TABLE concert
(
key SERIAL PRIMARY KEY,
club_name TEXT REFERENCES club(name) ON UPDATE CASCADE,
band_name TEXT REFERENCES band(name) ON UPDATE CASCADE,
concert_date DATE
);
In my issue, I had to define some additional operations (trigger) for updating the concert's table. Those operations had to modify club_name and band_name. I was unable to do it, because of reference. I couldn't modify concert and then deal with club and band tables. I couldn't also do it the other way. ON UPDATE CASCADE was the key to solve the problem.
The ON UPDATE and ON DELETE specify which action will execute when a row in the parent table is updated and deleted. The following are permitted actions : NO ACTION, CASCADE, SET NULL, and SET DEFAULT.
Delete actions of rows in the parent table
If you delete one or more rows in the parent table, you can set one of the following actions:
ON DELETE NO ACTION: SQL Server raises an error and rolls back the delete action on the row in the parent table.
ON DELETE CASCADE: SQL Server deletes the rows in the child table that is corresponding to the row deleted from the parent table.
ON DELETE SET NULL: SQL Server sets the rows in the child table to NULL if the corresponding rows in the parent table are deleted. To execute this action, the foreign key columns must be nullable.
ON DELETE SET DEFAULT: SQL Server sets the rows in the child table to their default values if the corresponding rows in the parent table are deleted. To execute this action, the foreign key columns must have default definitions. Note that a nullable column has a default value of NULL if no default value specified.
By default, SQL Server appliesON DELETE NO ACTION if you don’t explicitly specify any action.
Update action of rows in the parent table
If you update one or more rows in the parent table, you can set one of the following actions:
ON UPDATE NO ACTION: SQL Server raises an error and rolls back the update action on the row in the parent table.
ON UPDATE CASCADE: SQL Server updates the corresponding rows in the child table when the rows in the parent table are updated.
ON UPDATE SET NULL: SQL Server sets the rows in the child table to NULL when the corresponding row in the parent table is updated. Note that the foreign key columns must be nullable for this action to execute.
ON UPDATE SET DEFAULT: SQL Server sets the default values for the rows in the child table that have the corresponding rows in the parent table updated.
FOREIGN KEY (foreign_key_columns)
REFERENCES parent_table(parent_key_columns)
ON UPDATE <action>
ON DELETE <action>;
See the reference tutorial.
It's an excellent question, I had the same question yesterday. I thought about this problem, specifically SEARCHED if existed something like "ON UPDATE CASCADE" and fortunately the designers of SQL had also thought about that. I agree with Ted.strauss, and I also commented Noran's case.
When did I use it? Like Ted pointed out, when you are treating several databases at one time, and the modification in one of them, in one table, has any kind of reproduction in what Ted calls "satellite database", can't be kept with the very original ID, and for any reason you have to create a new one, in case you can't update the data on the old one (for example due to permissions, or in case you are searching for fastness in a case that is so ephemeral that doesn't deserve the absolute and utter respect for the total rules of normalization, simply because will be a very short-lived utility)
So, I agree in two points:
(A.) Yes, in many times a better design can avoid it; BUT
(B.) In cases of migrations, replicating databases, or solving emergencies, it's a GREAT TOOL that fortunately was there when I went to search if it existed.
My comment is mainly in reference to point #3: under what circumstances is ON UPDATE CASCADE applicable if we're assuming that the parent key is not updateable? Here is one case.
I am dealing with a replication scenario in which multiple satellite databases need to be merged with a master. Each satellite is generating data on the same tables, so merging of the tables to the master leads to violations of the uniqueness constraint. I'm trying to use ON UPDATE CASCADE as part of a solution in which I re-increment the keys during each merge. ON UPDATE CASCADE should simplify this process by automating part of the process.
To add to other great answers here it is important to use ON UPDATE CASCADE (or on DELETE CASCADE...) cautiously. Operations on tables with this specification require exclusive lock on underlaying relations.
If you have multiple CASCADE definitions in one table (as in other answer), and especially multiple tables using same definitions, and multiple users updating, this can create a deadlock when one process acquires exclusive lock on first underlaying table, other exclusive lock on second, and they block out each other by none of them being able to get both (all) exclusive locks to perform operation.