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.
Related
Here's the scenario.
- Parent Table: TEAMMEMBERS, with a primary key RecID
- Child Table: TEAMMEMBERTASKS
I have two columns in the TEAMMEMBERTASKS table, ReportedBy and AssignedTo.
Both of these columns use the RecID to store which team member reported a task and which team member the task is assigned to. The RecID could be the same for both columns, but that is not always the case.
I need to add in a FK for both child columns that check the relationship to the parent, and I would like to add ON UPDATE CASCADE to both of the foreign keys.
Whenever I try to do this, my second foreign key throws a 'may cause cycles or multiple cascade paths' error.
Here's my code:
ALTER TABLE [dbo].[TEAMMEMBERTASKS] WITH CHECK ADD CONSTRAINT
[FK_AssignedTo_TeamMemberRecID] FOREIGN KEY([AssignedTo])
REFERENCES [dbo].[TEAMMEMBERS] ([RecID])
GO
ALTER TABLE [dbo].[TEAMMEMBERTASKS] CHECK CONSTRAINT
[FK_AssignedTo_TeamMemberRecID]
GO
ALTER TABLE [dbo].[TEAMMEMBERTASKS] WITH CHECK ADD CONSTRAINT
[FK_ReportedBy_TeamMemberRecID] FOREIGN KEY([ReportedBy])
REFERENCES [dbo].[TEAMMEMBERS] ([RecID])
ON UPDATE CASCADE
GO
ALTER TABLE [dbo].[TEAMMEMBERTASKS] CHECK CONSTRAINT
[FK_ReportedBy_TeamMemberRecID]
GO
With the current code, will this cause the RecID to be updated in both child columns or will it cause the update command to be restricted?
Should I just go ahead and write up a trigger that deals with this instead?
Consider a situation where I define an object, a group of objects, then a table that links them together:
CREATE TABLE obj (
id INTEGER PRIMARY KEY,
name text
) ;
CREATE TABLE group (
id INTEGER PRIMARY KEY ;
grpname TEXT
) ;
CREATE TABLE relation (
objid INTEGER,
grpid INTEGER,
PRIMARY KEY (objid, grpid)
) ;
I am looking for cascade delete when applicable so I add the foreign key
ALTER TABLE relation
ADD FOREIGN KEY (objid)
REFERENCES obj(id)
ON DELETE CASCADE ;
ALTER TABLE relation
ADD FOREIGN KEY (grpid)
REFERENCES group(id)
ON DELETE CASCADE ;
So far is all OK. Now suppose I want to add support for group of groups. I am thinking to change the relation table like this:
CREATE TABLE relation_ver1 (
parent INTEGER,
child INTEGER,
PRIMARY KEY (parent, child)
) ;
ALTER TABLE relation_ver1
ADD FOREIGN KEY (parent)
REFERENCES group(id)
ON DELETE CASCADE ;
Here I get to the question: I would like to apply cascade delete to child too, but I do not know here if child refers to a group or object.
Can I add a foreign key to table obj or group?
The only solution I have found do fare is add child_obj and child_grp fields, add the relative foreign keys and then, when inserting e.g an object use a 'special' (sort of null) group, and do the reverse when inserting subgroup.
Consider the relation:
relation_ver1(parent, child_obj, child_group)
I claim that this relation has the following disadvantages:
You have to deal with the NULL special case.
Approx. 1/3 of values are NULL. NULL values are bad.
Fortunately, there is an easy way to fix this. Since there is a multi-value dependency in your data, you can decompose your table into 2 smaller tables that are 4NF compliant. For example:
relation_ver_obj(parent, child_obj) and
relation_ver_grp(parent, child_group).
The primary reason why we have foreign keys is not so as to be able to do things like cascaded deletes. The primary reason for the existence of foreign keys is referential integrity.
This means that grpid is declared as REFERENCES group(id) in order to ensure that grpid will never be allowed to take any value which is not found in group(id). So, it is an issue of validity. A cascaded DELETE also boils down to validity: if a key is deleted, then any and all foreign keys referring to that key would be left invalid, so clearly, something must be done about them. Cascaded deletion is one possible solution. Setting the foreign key to NULL, thus voiding the relationship, is another possible solution.
Your notion of having a child id refer to either a group or an object violates any notion of referential integrity. Relational Database theory has no use and no provision for polymorphism. A key must refer to one and only one kind of entity. If not, then you start running into problems like the one you have just discovered, but even worse, you cannot have any referential integrity guarantees in your database. That's not a nice situation to be in.
The way to handle the need of relationships to different kinds of entities is with the use of a set of foreign keys, one for each possible related entity, out of which only one may be non-NULL. So, here is how it would look like:
CREATE TABLE tree_relation (
parent_id INTEGER,
child_object_id INTEGER,
child_group_id INTEGER,
PRIMARY KEY (parent_id, child_object_id, child_group_id) );
ALTER TABLE tree_relation
ADD FOREIGN KEY (parent_id) REFERENCES group(id) ON DELETE CASCADE;
ALTER TABLE tree_relation
ADD FOREIGN KEY (child_object_id) REFERENCES object(id) ON DELETE CASCADE;
ALTER TABLE tree_relation
ADD FOREIGN KEY (child_group_id) REFERENCES group(id) ON DELETE CASCADE;
All you need to do is ensure that only one of child_object_id, child_group_id is non-NULL.
I want to delete a row/tuple from a parent table, but it is throwing an error message because it has a FOREIGN KEY reference in its child table.
However, in my case I want to delete the record only from the parent table and maintain the data in the child table.
Is it possible to achieve this?
I know the usage of ON DELETE CASCADE, but I want to know if there is a solution for the secenario I described?
It is possible with some agreements in your data. To maintain child table data you'll have to do ON DELETE SET NULL. This will leave data, but set FK to NULL value (in child table). And that is because of data-integrity: while you can keep your data, your FK can not refer to non-existent row of parent table in terms of enforcing FK constraint. Thus, it will be set to NULL by this.
If you want to "save" value of FK - then you definitely should not use FK at all because such behavior violates what FK is. So then just don't use that constraint, but be aware of possible integrity fails.
The point of a foreign key constraint is to prevent orphan records in the child table. So, no, it's not possible to do that, unless you drop the foreign key relationship.
If you rely on 'ON DELETE CASCADE', then deleting the parent record will result in all the corresponding children to be deleted.
If you want to delete the parent, but keep the children, you need to drop the foreign key constraint, or set the constraint to be 'ON DELETE SET NULL'. If you set 'ON DELETE SET NULL', then when you delete the parent record, the child records will remain, but the foreign key column value will be set to NULL.
delete a row ONLY in parent table, which is referenced by a Foregin Key from the child table
If Multiple table has been mapped in one table in that case all foreign key i.e :-
$table->integer('customer_id')->unsigned()->nullable();
$table->foreign('customer_id')->references('id')
->on('customers')->onDelete(`SET NULL`);
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 know the question of how to update multiple tables in SQL has been asked before and the common answer seems to be do them separately in a transaction.
However, the 2 columns I need to update have a foreign key so cannot be updated separately.
e.g.
Table1.a is a foreign key to Table2.a
One of the entries in the tables is wrong, e.g. both columns are 'xxx' and should be 'yyy'
How do I update Table1.a and Table2.a to be 'yyy'?
I know I could temp remove the key and replace but surely there's another way.
Thanks
You can't do the update simultaneously, however you can force SQL to do the update. You need to make sure your foreign keys have the referential triggered action ON UPDATE CASCADE
e.g.
ALTER TABLE YourTable
ADD CONSTRAINT FK_YourForeignKey
FOREIGN KEY (YourForeignKeyColumn)
REFERENCES YourPrimaryTable (YourPrimaryKeyColumn) ON UPDATE CASCADE
Not being a fan of on update cascade, I would suggest a different route.
First you do not update the Parent table, you add a new record with the value you want (and the same data as the other record for all other fields). Then you have no difficulty updating the child tables to use this value instead of that value. Further you now have the ability to to do the work in batches to avoid locking the system up while the change promulgates through it. Once all the child tables have been updated, you can delete the original bad record.
my answer is based on the following link: http://msdn.microsoft.com/en-us/library/ms174123%28v=SQL.90%29.aspx
You need to make sure that your table_constraint will be defined as ON UPDATE CASCADE
CREATE TABLE works_on1
(emp_no INTEGER NOT NULL,
project_no CHAR(4) NOT NULL,
job CHAR (15) NULL,
enter_date DATETIME NULL,
CONSTRAINT prim_works1 PRIMARY KEY(emp_no, project_no),
CONSTRAINT foreign1_works1 FOREIGN KEY(emp_no) REFERENCES employee(emp_no) ON DELETE CASCADE,
CONSTRAINT foreign2_works1 FOREIGN KEY(project_no) REFERENCES project(project_no) ON UPDATE CASCADE)
and then when you will change the value of your primary key
see the following quote:
For ON DELETE or ON UPDATE, if the CASCADE option is specified, the
row is updated in the referencing table if the corresponding
referenced row is updated in the parent table. If NO ACTION is
specified, SQL Server Compact Edition returns an error, and the update
action on the referenced row in the parent table is rolled back.
For example, you might have two tables, A and B, in a database. Table
A has a referential relationship with table B: the A.ItemID foreign
key references the B.ItemID primary key.
If an UPDATE statement is executed on a row in table B and an ON
UPDATE CASCADE action is specified for A.ItemID, SQL Server Compact
Edition checks for one or more dependent rows in table A. If any
exist, the dependent rows in table A are updated, as is the row
referenced in table B.
Alternatively, if NO ACTION is specified, SQL Server Compact Edition
returns an error and rolls back the update action on the referenced
row in table B when there is at least one row in table A that
references it.