Is it possible to have both foreign key constraint and a trigger that keep the referential integrity?
I don't think that is possible and we have to remove the constraint and keep the referential integrity using trigger. To extend my description, suppose you have two tables one of them has a primary key and the other has a foreign key referring to the first table, primary key column. So now is it possible to add the following code
create or replace TRIGGER TABLE1_TABLE2
after update of TABLE1_PRIMARY_KEY on TABLE1
Of course it came to my mind to lose the constraint and continue with the trigger but is it the only way?
By the way this code is just for practice, I know it isn't a good idea to update the primary key of a table.
I found a useful article on one of Microsoft websites confirming my idea that the constraints should be removed. But of course the solution for Oracle may be different from any other database. The article is about Implementing Referential Integrity and Cascading Actions and the part that is related is "Implementing ON DELETE CASCADE and ON UPDATE CASCADE with triggers" and after that comes "Before continuing, you need to drop the existing foreign keys".
There is no problem doing that in Oracle. You could have both the constraint and the trigger at the same time.
Related
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 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 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
I have two tables in mysql, both are type InnoDB and there is an existing foreign key from the first to the second.
However, the foreign key was created without any cascading behaviour. The table is large, and any changes to indexes or new keys locks the table for ~20 minutes.
Is it possible to add "ON DELETE CASCADE" behaviour to an existing foreign key without dropping the key and recreating it?
Unfortunately, no. On Delete and On Update are systemic attributes of the foreign key itself and can only be specified when creating the constraint.
Does anyone know if there is a quicker way of editing a record that has foreign keys in a table (in sql server).. i will explain.. i have approx 5 tables that have there own ID but are linked together using a foreign key...
Hence i needed to change the foreign key (the contract number in my case), but i had to copy each record to a new record and edit it that way...
As if i try to edit the contract number it gives me the standard error of being associated and violates a foreign key etc
Surly there must be a better way?
ANy ideas?
are you talking about changing the PK and then updating all the Fks? In that case enable cascade updates and this will be done automagically
same with deletes, you enable cascade deletes
ON DELETE CASCADE
Specifies that if an attempt is made to delete a row with a key referenced by foreign keys in existing rows in other tables, all rows containing those foreign keys are also deleted. If cascading referential actions have also been defined on the target tables, the specified cascading actions are also taken for the rows deleted from those tables.
ON UPDATE CASCADE
Specifies that if an attempt is made to update a key value in a row, where the key value is referenced by foreign keys in existing rows in other tables, all of the foreign key values are also updated to the new value specified for the key. If cascading referential actions
I'm not an SQL expert, but can't you set something like ON UPDATE CASCADE to automatically update the foreign key when the primary key is changed?
Or try disabling the integrity constraint, do your changes and attempt to re-enable the constraint. Basically, if you didn't do it right you will get an error then (can't enable a constraint that would be violated).