I have two tables, Games and Sport_Games. Sport_Games has a foreign key that references Games.
Now when I want to delete an entry from Games I get an error saying:
"FK__Sport_Gam__game___1D7C2B7C". The conflict occurred in database "DatabasesProject", table "dbo.Sport_Games", column 'game_id'.
The statement has been terminated.
Im assuming this is because I can't delete something from a table with a constraint without dropping it. I tried adding ON DELETE CASCADE in table Games in hopes that when I delete a game it is also deleted from Sport_Games but that didn't work either.
Here are the tables for reference:
CREATE TABLE Games(
game_id INT IDENTITY PRIMARY KEY,
name VARCHAR(50),
release_date date,
rating INT,
min_age INT,
development_team_email VARCHAR(50) FOREIGN KEY REFERENCES Development_Teams,
release_conference INT FOREIGN KEY REFERENCES Conferences
--ON DELETE CASCADE ---this is what I added
)
CREATE TABLE Sport_Games(
game_id INT PRIMARY KEY FOREIGN KEY REFERENCES Games,
sport_type VARCHAR(50),
)
I have other tables referencing Games as well, action_games, strategy_games, But they're similar to sport_games
If you know the GameID of the records you are deleting, simply delete records in the Sport_Games table that have the same GameID first, then delete from Games table.
For the cascade to work, you need to ad that to the FK definition on the Sport_Games table. That way when the Games record is deleted, the sport_Games record will be deleted as well.
Your ON DELETE CASCADE will work for you if you put it on the correct Foreign Key. But to answer your question...
Since Games.Game_ID has a constraining reference against Sport_Games.Game_ID, you need to either turn the constraint off so it will not prevent you from deleting, but this will leave orphans, or delete the children before you delete the parent. Since you must already know the PK value to delete the parent, you simply:
DELETE FROM Sport_Games WHERE Game_ID = [The id you are using to delete from Games]
Then you delete the parent:
DELETE FROM Games WHERE Game_ID = [Same id used above]
What I'm about to say might not directly answer your question, but it could be beneficial for you moving forward or in future designs.
I've found placing "delete_date" and "delete_id" in tables to be a useful way to work around these FK constraint issues. If your application is only calling existing stored procedures and table functions you could update the queries to include "AND delete_date IS NULL" in their conditions. If your application is writing adhoc queries, it might also be as simple as creating or modifying a base class to inject the condition that omits soft-deleted rows.
Related
I create 3 tables just like image. Each students can be enrolled multiple class I tried to build one to many relation.
What I want to do is, when a student is deleted from the "Student" table, the course in which the student is registered in the "Bridge" table returns to null. How can I do this operations with postgresql (pgAdmin 4), can you help me please? Thank you...
You are describing the on delete set null option to foreign keys constraints. The create table statement for bridge would look like:
create table bridge (
std_id int references students(std_id) on delete set null,
class_id int references class(class_id)
);
I am unsure that set null is your best pick for such a bridge table though. This leaves "gaps" in your data that do not make a lot of sense. on delete cascade would probably make more sense - and you could apply it to both foreign keys:
create table bridge (
std_id int references students(std_id) on delete cascade,
class_id int references class(class_id) on delete cascade
);
That way, the bridge table is properly cleaned up when any parent record is dropped. This also opens the way to set up a composite primary key made of both columns in the bridge table.
I have a table which is referenced by multiple tables (around 52) and further,few of the child tables have multiple foreign keys also that is referencing other tables too.
I want to delete a record from parent table, I am unable to do so, as I am getting error "The DELETE statement conflicted with the REFERENCE constraint "FK_xxx". The conflict occurred in database "MyDB", table "dbo.A", column 'x'."
I want a generalized T-SQL solution which is irrespective of tables and number of references.
You have to look at the "on delete" keyword which is a part of the foreign key constraint definition.
Basically you have 4 options:
NO ACTION (does nothing)
CASCADE (deletes the child aswell)
SET NULL (sets the reference field to null)
SET DEFAULT (sets the reference field to the default value)
An example would be:
CREATE TABLE parent (
id INT NOT NULL,
PRIMARY KEY (id)
) ENGINE=INNODB;
CREATE TABLE child (
id INT,
parent_id INT,
INDEX par_ind (parent_id),
FOREIGN KEY (parent_id)
REFERENCES parent(id)
ON DELETE CASCADE -- replace CASCADE with your choice
) ENGINE=INNODB;
(for this example and more details look here: http://dev.mysql.com/doc/refman/5.7/en/create-table-foreign-keys.html )
If you now want to modify your constraint, you first have to drop it, and create a new one like for example:
ALTER TABLE child
ADD CONSTRAINT fk_name
FOREIGN KEY (parent_id)
REFERENCES parent(id)
ON DELETE CASCADE; -- replace CASCADE with your choice
I hope this helped. Also to mention it, you should think about maybe not really deleting your parent, and instead creating another boolean column "deleted", which you fill with "yes" if someone clicks the delete. In the "Select"-query you filter then by that "deleted" column.
The advantage is, that you do not lose the history of this entry.
Your problem is this: A FK constraint is designed to prevent you from creating an orphaned child record in any of the 52 tables. I can provide you with the script you seek, but you must realise first that when you try to re-enable the FK constraints the constraints will fail to re-enable because of the orphaned data (which the FK constraints are designed to prevent). For your next step, will have to delete the orphaned data in each of the 52 tables first anyway. It is actually much easier just to redo the constraints with ON DELETE CASCADE, or drop the constraints and forget about referential integrity altogether. You can't have it both ways.
I have a sql table (let's call it Person) like, for example :
TABLE Person
Id Firstname Lastname
I want to make relations beetwen somme of this person (parent/child relation) so I built another table (let's call it Person_Relation) like that :
TABLE Person_Relation
Id_person_parent Id_person_child
I made a constraint to avoid the case where parent = child (it would be awkward !) and now i try to make foreign key beetwen table Person and table Person_Relation.I am currently able to make one foreign key, but when i try to set the second I get a : may cause cycles or multiple cascade paths error.
Knowing that I would keep the 'Delete cascade' to automatically delete links in Person_Relation table when an entry in Person is deleted, is there any clean solution to do that ?
Thank you in advance.
SQL Server won't let you create multiple cascade paths that could theoretically conflict. For more on that, see this answer.
One way to still achieve your goal is to use a trigger in place of a foreign key with a cascade action.
CREATE TRIGGER dbo.Person_Delete
ON dbo.Person
FOR DELETE
AS
DELETE dbo.Person_Relation
WHERE Id_person_parent IN (SELECT Id FROM deleted)
OR Id_person_child IN (SELECT Id FROM deleted);
What's the difference? If I have these two tables:
CREATE TABLE Account (Id int NOT NULL)
CREATE TABLE Customer (AccountId int NOT NULL)
And I want a foreign key linking the two, which of the following should I do and why?
Option 1:
ALTER TABLE [dbo].[Customer] WITH CHECK
ADD CONSTRAINT [FK_Accounts_Customers] FOREIGN KEY([AccountId])
REFERENCES [dbo].[Account] ([Id])
Option 2:
ALTER TABLE [dbo].[Account] WITH CHECK
ADD CONSTRAINT [FK_Accounts_Customers] FOREIGN KEY([Id])
REFERENCES [dbo].[Customer] ([Id])
Depends on context. Does every customer have a client? Which one is the parent? It seems like an Account has multiple Customers, in which case the reference belongs on the Customer table.
Now, that said, please call the entities CustomerID and AccountID everywhere. It may seem redundant on the primary table but the name should be consistent throughout the model.
I would use a foreign key from the child to the parent. The tell tale question is: what happens if you need to delete one of the entities?
A FK (foreign key) tells the DBMS that values for subrows for a column list must appear elsewhere as values for subrows for a column list. Whenever that happens (and it isn't already implied by other declartions) declare the FK. If in addtion you want a CASCADE action applied to the referenced table on a change to the referencing table, declare that.
(There's nothing special about CASCADE that it couldn't be offered for non-FK situations. It just comes up frequently with FKs, and there's an explicit graph of FKs by which to reasonably restrict their interactions.)
If there is a FK cycle then you will need to use triggers. Your decision of which constraint(s) are enforced declaratively & which by trigger should consider the graph of (desired) constraints.
I have 2 tables:
1. Employees
2. Vouchers
Employees table has a single primary key.
Vouchers table has 3 foreign key constraints referencing the Employees table.
The following is a sample T-SQL script (not the actual table script) to create both tables and their relationship in SQL Server:
IF OBJECT_ID('dbo.Vouchers') IS NOT NULL
DROP TABLE dbo.Vouchers
IF OBJECT_ID('dbo.Employees') IS NOT NULL
DROP TABLE dbo.Employees
GO
CREATE TABLE Employees
(
ObjectID INT NOT NULL PRIMARY KEY IDENTITY
)
CREATE TABLE Vouchers
(
ObjectID INT NOT NULL PRIMARY KEY IDENTITY,
IssuedBy INT,
ReceivedBy INT,
ApprovedBy INT,
CONSTRAINT fk_Vouchers_Employees_IssuedBy FOREIGN KEY (IssuedBy)
REFERENCES Employees (ObjectID)
ON UPDATE CASCADE
ON DELETE NO ACTION,
CONSTRAINT fk_Vouchers_Employees_ReceivedBy FOREIGN KEY (ReceivedBy)
REFERENCES Employees (ObjectID)
ON UPDATE CASCADE
ON DELETE NO ACTION,
CONSTRAINT fk_Vouchers_Employees_ApprovedBy FOREIGN KEY (ApprovedBy)
REFERENCES Employees (ObjectID)
ON UPDATE CASCADE
ON DELETE NO ACTION
)
But an error is thrown:
Msg 1785, Level 16, State 0, Line 7
Introducing FOREIGN KEY constraint 'fk_Vouchers_Employees_ReceivedBy' on table 'Vouchers' may cause cycles or multiple cascade paths. Specify ON DELETE NO ACTION or ON UPDATE NO ACTION, or modify other FOREIGN KEY constraints.
I don't have an idea of what efficient solution is available here. The requirements on the relationship is that: whenever an Employee is deleted, the Voucher that references some of its columns to the Employee does not get deleted (ON DELETE CASCADE is not an option). Instead, the values of the columns (IssuedBy, ReceivedBy and/or ApprovedBy) that are referenced to the deleted Employee should be set to NULL (since the columns are NULLABLE).
Many thanks!
Strictly from a relational design point of view, the Vouchers table as three Foreign Keys. Whether you choose to enforce them, through CASCADE assertions or otherwise, is an implementation issue, but the relational design still exists. Presumably you want to enforce that, if one of the three fields is not NULL, then a matching record needs to exist. Or not. It's an implementation issue as to whether or not you care to enforce the design.
However, the violations you describe are assumed at your peril. The fact that you're asking this question suggests you may not fully appreciate all the ways these choices can lead into the swamp.
I think the error may be a consequence of the fact that more than one of the three might refer to the same employee.
BTW, I've in very few cases ever found it necessary to delete records in such a fashion that CASCADES are useful. Usually that would be used to prevent the database from being too big; and database capacity is less and less an issue over time.
I would not actually delete Employees, but instead use a trigger to set a flag to mark them as deleted.
I generally don't turn on cascade of Updates or Deletes, but instead require an application to explicitly perform these actions.
From a design standpoint it seems good to have the 3 foreign keys you listed. It looks like the error message you are getting relates to the ON UPDATE CASCADE options on your foreign keys (although I was able to create the table as specified). Regardless, to get the behavior you mention wanting, I would recommend a trigger on the Employees table, that fires before you delete the record. This trigger would find instances of the Employees.OjbectID in the Vouchers table and set them to NULL.