If I had two tables where I need to add a new foreign key to, say:
ALTER TABLE [release_user]
ADD CONSTRAINT [user_release]
FOREIGN KEY ([user_id]) REFERENCES [user_table] ([user_id])
ON DELETE CASCADE
If there are records in the release_user table that do not have a matching ID in the user_table, the command will fail. I expect that.
However, is there a way to create the FK, and have it automatically delete (DELETE CASCADE) the records that would cause the FK to fail?
(Or, for example, if I used DELETE SET NULL, would null out the record's field in the release_user table?)
You could delete them first?
DB_FIDDLE
delete from ru
from release_user ru
left join user_table ut
on ut.user_id = ru.user_id
where ut.user_id is null
Then, add your constraint.
Can try by using trigger as below
(can refer the actual syntax below sample just gives the logic via trigger )
Create Trigger delete_check
Before Insert on
release_user
For EACH Row
If( Select count( * ) from
inserted where :New.ID NOT In
(Select Id from user_table ) >=1)
{ dbms_output.put_line("dont insert")
either
< delete from release_user where
id =:new.id>
or <insert into user_table
values(:New.id)> (to avoid
constraint error on insert)
}
END
Related
I have table on which I have a foreign key constraint like below
ALTER TABLE [dbo].[Element] WITH CHECK
ADD CONSTRAINT [FK_Element_Band]
FOREIGN KEY([BandID]) REFERENCES [dbo].[Band] ([BandID])
GO
ALTER TABLE [dbo].[Element] CHECK CONSTRAINT [FK_Element_Band]
GO
Now I am trying to delete a row from the band table like this
DELETE FROM Band
WHERE TypeID = 21 AND BandUpperLimit = 10000 AND PID = 61
But I am getting a reference constraint error:
The DELETE statement conflicted with the REFERENCE constraint "FK_Element_Band". The conflict occurred in database "pricingModified", table "dbo.Element", column 'BandID'.
So this should happen if I have anything in the Elements table which is referencing Band table with the ID 21 but that's not the case since the SQL
SELECT *
FROM Element
WHERE BandID = 21
returns nothing.
Can someone please tell me why I am not able to delete row data from the parent table even though there is no reference present in the child table?
Thanks
Just specify ON DELETE CASCADE in your foreign key constraint. Then when you delete a row from the Band table it will automatically delete any child records in the Element table.
ALTER TABLE [dbo].[Element] WITH CHECK
ADD CONSTRAINT [FK_Element_Band]
FOREIGN KEY([BandID]) REFERENCES [dbo].[Band] ([BandID]) ON DELETE CASCADE
GO
Check with this query instead:
select b.*
from Element e
inner join Band b
on e.BandId = b.BandId
where b.TypeId = 21
and b.BandUpperLimit = 10000
and b.PID = 61
while creating child table
create table <child_table_name>
(
_______,
_______,
([FOREIGN KEY column_name]) [size of FOREIGN KEY column] references [parent table(primary key column_name)] on delete cascade
)
Using this you can easily delete record of fk using delete operation..
Try this, if it is successful then mark it positive...
I have a comment table that is self-referencing.
I tried to write on delete cascade but it take some exception
Introducing FOREIGN KEY constraint 'FK_Comments_Comments' on table 'Comments' may cause cycles or multiple cascade paths. Specify ON DELETE NO ACTION or ON UPDATE NO ACTION, or modify other FOREIGN KEY constraints.
and then try to write a trigger but it take exception again
CREATE TRIGGER [dbo].[T_comment_Trigger]
ON [dbo].[Comments]
FOR DELETE
AS
DELETE FROM Comments
WHERE ParentId =(SELECT deleted.id FROM deleted)
couldn't delete rows that have children
how can I do on delete cascade for my self-referencing table?
Assuming you're keeping your FOREIGN KEY constraint in place, you cannot fix the issue in a FOR DELETE trigger. FOR triggers (also known as AFTER triggers) fire after the activity has taken place. And a foreign key will prevent a row from being deleted if it has references. Foreign key checks occur before deletion.
What you need is an INSTEAD OF trigger. You also need to bear in mind that your current trigger only tried to deal with one "level" of referencing. (So, if row 3 references row 2 and row 2 references row 1, and you delete row 1, your trigger only tried to remove row 2)
So, something like:
CREATE TRIGGER [dbo].[T_comment_Trigger]
ON [dbo].[Comments]
INSTEAD OF DELETE
AS
;WITH IDs as (
select id from deleted
union all
select c.id
from Comments c
inner join
IDs i
on
c.ParentID = i.id
)
DELETE FROM Comments
WHERE id in (select id from IDs);
If there are other (non-self-referencing) cascading foreign key constraints, they all have to be replaced by actions in this trigger. In such a case, I'd recommend introducing a table variable to hold the list of all IDs that will eventually be deleted from the Comments table:
CREATE TRIGGER [dbo].[T_comment_Trigger]
ON [dbo].[Comments]
INSTEAD OF DELETE
AS
declare #deletions table (ID varchar(7) not null);
;WITH IDs as (
select id from deleted
union all
select c.id
from Comments c
inner join
IDs i
on
c.ParentID = i.id
)
insert into #deletions(ID)
select ID from IDs
-- Delete from FK referenced table
DELETE FROM OtherTable
WHERE CommentID in (select ID from #deletions)
--This delete comes last
DELETE FROM Comments
WHERE id in (select ID from #deletions);
Create Trigger:
SELECT #oldVersionId = (SELECT DISTINCT VERSION_ID FROM Deleted)
SELECT #newVersionId = (SELECT DISTINCT VERSION_ID FROM Inserted)
SELECT #appId = (SELECT DISTINCT APP_ID FROM Deleted)
UPDATE [TableName]
SET [VERSION_ID] = #newVersionId
WHERE (([VERSION_ID] = #oldVersionId) AND ([APP_ID] = #appId) )
Can this Trigger be replace with a Foreign Key to update the VERSION_ID ?
What I think could be a problem is the AND condition, how to express that in a FK with On del/update Cascade?
FOREIGN KEY CONSTRAINTS don't update anything. They check the values being written to a record and cause the write to fail if they cause a constraint to fail.
Also, as #marc_s points out in his comment, triggers in MS SQL Server are set based. The INSERTED and DELETED tables can hold multiple records at once. Your code only works for one record.
You could try something along these lines...
UPDATE
table
SET
VERSION_ID = inserted.VERSION_ID
FROM
table
INNER JOIN
deleted
ON table.VERSION_ID = deleted.VERSION_ID
AND table.APP_ID = deleted.APP_ID
INNER JOIN
inserted
ON deleted.PRIMARY_KEY = inserted.PRIMARY_KEY
EDIT
I just read your comment, and I think I understand. You want a foreign key constraint with ON UPDATE CASCADE.
You use this format to create that with DDL.
ALTER TABLE DBO.<child table>
ADD CONSTRAINT <foreign key name> FOREIGN KEY <child column>
REFERENCES DBO.<parent table>(<parent column>)
{ON [DELETE|UPDATE] CASCADE}
Or you could just SQL Server Management Studio to set it up. Just make sure the ON UPDATE CASCADE is present.
I cannot really tell you what you're looking for - you're too unclear in your question.
But basically, if two tables are linked via a foreign key constraint, of course you can add a clause to that to make sure the child table gets updated when the parents table's PK changes:
ALTER TABLE dbo.ChildTable
ADD CONSTRAINT FK_ChildTable_ParentTable
FOREIGN KEY(ChildTableColumn) REFERENCES dbo.ParentTable(PKColumn)
ON UPDATE CASCADE
The ON UPDATE CASCADE does exactly that - if the referenced column (the PKColumn in ParentTable) changes, then the FK constraint will "cascade" that update down into the child table and update it's ChildTableColumn to match the new PKColumn
Read all about cascading referential integrity constraints and what options you have on MSDN Books Online
what is the recommended way to handle self-referencing foreignkey constraints in SQL-Server?
Table-Model:
fiData references a previous record in tabData. If i delete a record that is referenced by fiData, the database throws an exception:
"The DELETE statement conflicted with the SAME TABLE REFERENCE
constraint "FK_tabDataPrev_tabDataNext". The conflict occurred in
database "MyDataBase", table "dbo.tabData", column 'fiData'"
if Enforce Foreignkey Constraint is set to "Yes".
I don't need to cascade delete records that are referenced but i would need to set fiData=NULL where it's referenced. My idea is to set Enforce Foreignkey Constraint to "No" and create a delete-trigger. Is this recommendable or are there better ways?
Thank you.
Unlike Andomar, I'd be happy using a trigger - but I wouldn't remove the constraint checking. If you implement it as an instead of trigger, you can reset the other rows to null before performing the actual delete:
CREATE TRIGGER T_tabData_D
on tabData
instead of delete
as
set nocount on
update tabData set fiData = null where fiData in (select idData from deleted)
delete from tabData where idData in (select idData from deleted)
It's short, it's succinct, it wouldn't be necessary if SQL Server could handle foreign key cascades to the same table (in other RDBMS', you may be able to just specify ON DELETE SET NULL for the foreign key constraint, YMMV).
Triggers add implicit complexity. In a database with triggers, you won't know what a SQL statement does by looking at it. In my experience triggers are a bad idea with no exceptions.
In your example, setting the enforced constrained to "No" means you could add a nonexistent ID. And the query optimizer will be less effective because it can't assume the key is valid.
Consider creating a stored procedure instead:
create procedure dbo.NukeTabData(
#idData int)
as
begin transaction
update tabData set fiData = null where fiData = #idData
delete from tabData where idData = #idData
commit transaction
go
This very late to answer.
But for some one who is searching like me.
and want to cascade
here is very good explanation
http://devio.wordpress.com/2008/05/23/recursive-delete-in-sql-server/
The Problem
Although you can define a foreign key with CASCADE DELETE in SQL Server, recursive cascading deletes are not supported (i.e. cascading delete on the same table).
If you create an INSTEAD OF DELETE trigger, this trigger only fires for the first DELETE statement, and does not fire for records recursively deleted from this trigger.
This behavior is documented on MSDN for SQL Server 2000 and SQL Server 2005.
The Solution
Suppose you have a table defined like this:
CREATE TABLE MyTable (
OID INT, -- primary key
OID_Parent INT, -- recursion
... other columns
)
then the delete trigger looks like this:
CREATE TRIGGER del_MyTable ON MyTable INSTEAD OF DELETE
AS
CREATE TABLE #Table(
OID INT
)
INSERT INTO #Table (OID)
SELECT OID
FROM deleted
DECLARE #c INT
SET #c = 0
WHILE #c <> (SELECT COUNT(OID) FROM #Table) BEGIN
SELECT #c = COUNT(OID) FROM #Table
INSERT INTO #Table (OID)
SELECT MyTable.OID
FROM MyTable
LEFT OUTER JOIN #Table ON MyTable.OID = #Table.OID
WHERE MyTable.OID_Parent IN (SELECT OID FROM #Table)
AND #Table.OID IS NULL
END
DELETE MyTable
FROM MyTable
INNER JOIN #Table ON MyTable.OID = #Table.OID
GO
Is it true that MS SQL restrict self-referencing constraints with ON DELETE CASCADE option?
I have a table with parent-child relation, PARENT_ID column is foreign key for ID. Creating it with ON DELETE CASCADE option causes error
"Introducing FOREIGN KEY constraint
may cause cycles or multiple cascade
paths. Specify ON DELETE NO ACTION or
ON UPDATE NO ACTION, or modify other
FOREIGN KEY constraints."
I can't believe that I have to delete this hierarchy in recursive mode. Is there any issue except triggers?
It is the case that you cannot set up ON DELETE CASCADE on a table with self-referencing constraints. There is a potential of cyclical logic problems, hence it won't allow it.
There's a good article here - though it's for version 8 rather than 9 of SQL - though the same rules apply.
I just answered another question where this question was bound as duplicate. I think it's worth to place my answer here too:
This is not possible. You can solve this with an INSTEAD OF TRIGGER
create table locations
(
id int identity(1, 1),
name varchar(255) not null,
parent_id int,
constraint pk__locations
primary key clustered (id)
)
GO
INSERT INTO locations(name,parent_id) VALUES
('world',null)
,('Europe',1)
,('Asia',1)
,('France',2)
,('Paris',4)
,('Lyon',4);
GO
--This trigger will use a recursive CTE to get all IDs following all ids you are deleting. These IDs are deleted.
CREATE TRIGGER dbo.DeleteCascadeLocations ON locations
INSTEAD OF DELETE
AS
BEGIN
WITH recCTE AS
(
SELECT id,parent_id
FROM deleted
UNION ALL
SELECT nxt.id,nxt.parent_id
FROM recCTE AS prv
INNER JOIN locations AS nxt ON nxt.parent_id=prv.id
)
DELETE FROM locations WHERE id IN(SELECT id FROM recCTE);
END
GO
--Test it here, try with different IDs. You can try WHERE id IN(4,3) also...
SELECT * FROM locations;
DELETE FROM locations WHERE id=4;
SELECT * FROM locations
GO
--Clean-Up (Carefull with real data!)
if exists(select 1 from INFORMATION_SCHEMA.TABLES where TABLE_NAME='locations')
---DROP TABLE locations;
CREATE TRIGGER MyTable_OnDelete ON MyTable
INSTEAD OF DELETE
AS
BEGIN
SET NOCOUNT ON;
DELETE FROM mt
FROM deleted AS D
JOIN MyTable AS mt
ON d.Id = mt.ParentId
DELETE FROM mt
FROM deleted AS D
JOIN MyTable AS mt
ON d.Id = mt.Id
END