I have a fairly simple design, as follows:
What I want to achieve in my grouping_individual_history is marked in red:
when a session is deleted, I want to cascade delete the grouping_history....
when a grouping is deleted, I just want the child field to be nullified
It seems that MSSQL will not allow me to have more than one FK that does something else than no action ... It'll complain with:
Introducing FOREIGN KEY constraint 'FK_grouping_individual_history_grouping' on table 'grouping_individual_history' may cause cycles or multiple cascade paths. Specify ON DELETE NO ACTION or ON UPDATE NO ACTION, or modify other FOREIGN KEY constraints.
I've already read this post (https://www.mssqltips.com/sqlservertip/2733/solving-the-sql-server-multiple-cascade-path-issue-with-a-trigger/), although it's not quite the same scenario it seems to me.
I've tried doing a INSTEAD OF DELETE trigger on my grouping table, but it wont accept it, because in turn, my grouping table has another FK (fkSessionID) that does a cascade delete... So, the fix would be to change it all, in all affected tables with FKs. The chain is long though, and we cannot consider it.
For one thing, can someone explain to me why SQL Server is giving me the issue for this very simple scenario in the first place? I just don't understand it.
Is there another workaround I could use (besides just removing the foreign key link from my grouping_individual_history table)?
I went over a legacy database and found a couple of foreign keys that reference a column to itself. The referenced column is the primary key column.
ALTER TABLE [SchemaName].[TableName] WITH CHECK ADD
CONSTRAINT [FK_TableName_TableName] FOREIGN KEY([Id])
REFERENCES [SchemaName].[TableName] ([Id])
What is the meaning of it?
ALTER TABLE [SchemaName].[TableName] WITH CHECK ADD
CONSTRAINT [FK_TableName_TableName] FOREIGN KEY([Id])
REFERENCES [SchemaName].[TableName] ([Id])
This foreign key is completely redundant and pointless just delete it. It can never be violated as a row matches itself validating the constraint.
In a hierarchical table the relationship would be between two different columns (e.g. Id and ParentId)
As for why it may have been created quite likely through use of the visual designer if you right click the "Keys" node in object explorer and choose "New Foreign Key" then close the dialogue box without deleting the created foreign key and then make some other changes in the opened table designer and save it will create this sort of redundant constraint.
In some cases this is a preferred way to reduce redundancy in your model. In using the self referencing foreign key (as shown in you example) you create a hierarchical relationship between rows in your table. Pay attention to what happens when you delete a row from the table, cascading on delete might remove rows you still want.
Using these sort of keys moves some of the data validation to the DB model as opposed to making this a responsibility of the program/programmer. Some outfits prefer this way of doing things. I prefer to make sure programs and programmers are responsible - data models can be hard to refactor and upgrade in production environments.
I've been using table associations with SQL (MySQL) and Rails without a problem, and I've never needed to specify a foreign key constraint.
I just add a table_id column in the belongs_to table, and everything works just fine.
So what am I missing? What's the point of using the foreign key clause in MySQL or other RDBMS?
Thanks.
A foreign key is a referential constraint between two tables
The reason foreign key constraints exist is to guarantee that the referenced rows exist.
The foreign key identifies a column or set of columns in one (referencing or child) table that refers to a column or set of columns in another (referenced or parent) table.
you can get nice "on delete cascade" behavior, automatically cleaning up tables
There are lots of reason of using foreign key listed over here: Why Should one use foreign keys
Rails (ActiveRecord more specifically) auto-guesses the foreign key for you.
... By default this is guessed to be the name of the association with an “_id” suffix.
Foreign keys enforce referential integrity.
Foreign key: A column or set of columns in a table whose values are required to match at least one PrimaryKey values of a row of another table.
See also:
http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#method-i-belongs_to
http://c2.com/cgi/wiki?ForeignKey
The basic idea of foreign keys, or any referential constraint, is that the database should not allow you to store obviously invalid data. It is a core component of data consistency, one of the ACID rules.
If your data model says that you can have multiple phone numbers associated with an account, you can define the phone table to require a valid account number. It's therefore impossible to store orphaned phone records because you cannot insert a row in the phone table without a valid account number, and you can't delete an account without first deleting the phone numbers. If the field is birthdate, you might enforce a constraint that the date be prior to tomorrow's date. If the field is height, you might enforce that the distance be between 30 and 4000 cm. This means that it is impossible for any application to store invalid data in the database.
"Well, why can'd I just write all that into my application?" you ask. For a single-application database, you can. However, any business with a non-trivial database that stores data used business operations will want to access data directly. They'll want to be able to import data from finance or HR, or export addresses to sales, or create application user accounts by importing them from Active Directory, etc. For a non-trivial application, the user's data is what's important, and that's what they will want to access. At some point, they will want to access their data without your application code getting in the way. This is the real power and strength of an RDMBS, and it's what makes system integration possible.
However, if all your rules are stored in the application, then your users will need to be extremely careful about how they manipulate their database, lest they cause the application to implode. If you specify relational constraints and referential integrity, you require that other applications modify the data in a way that makes sense to any application that's going to use it. The logic is tied to the data (where it belongs) rather than the application.
Note that MySQL is absolute balls with respect to referential integrity. It will tend to silently succeed rather than throw errors, usually by inserting obviously invalid values like a datetime of today when you try to insert a null date into a datetime field with the constraint not null default null. There's a good reason that DBAs say that MySQL is a joke.
Foreign keys enforce referential integrity. Foreign key constraint will prevent you or any other user from adding incorrect records by mistake in the table. It makes sure that the Data (ID) being entered in the foreign key does exists in the reference table. If some buggy client code tries to insert incorrect data then in case of foreign key constraint an exception will raise, otherwise if the constraint is absent then your database will end up with inconsistent data.
Some advantages of using foreign key I can think of:
Make data consistent among tables, prevent having bad data( e.g. table A has some records refer to something does not exist in table B)
Help to document our database
Some framework is based on foreign keys to generate domain model
This is a 2 part question.
Question 1: I am trying to create a foreign key on a table where I need to turn off the "Check Existing Data on Creation or Re-Enabling". I know theres an option visually but I'm looking for a way to do it programmatically. Is there anyway to do this?
Question 2: I have a code table and two tables A and B that need to reference that code table. I want to have these both referenced from a relationship table but I want to able to use the same column. Can I have 2 foreign keys pointing to the same column?
Yes you can have the same column inthe parent table refer to differnt columns in multiple tables.
I do not recommend turning off checking FK on creation. If you have bad data now, you need to fix it now. Otherwise the first time someone edits one of those records it will fail the FK check then.
From Books online as to why it is a bad idea to use nocheck:
If you do not want to verify new CHECK
or FOREIGN KEY constraints against
existing data, use WITH NOCHECK. We do
not recommend doing this, except in
rare cases. The new constraint will be
evaluated in all later data updates.
Any constraint violations that are
suppressed by WITH NOCHECK when the
constraint is added may cause future
updates to fail if they update rows
with data that does not comply with
the constraint.
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.