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SQL Server Referential Integrity without Foreign Keys


By:   |   Last Updated: 2016-03-31   |   Comments (1)   |   Related Tips: More > Constraints

Problem

Referential integrity is a set of rules that enforces a relationship between data in tables and is aimed at keeping SQL Server databases consistent. Usually referential integrity is implemented by using foreign key constraints, however there can be situations when it is not possible to create a foreign key constraint and we need to find alternate ways to enforce referential integrity. In this article we will investigate solutions for implementing referential integrity other than foreign keys.

Solution

Assume a situation when two tables are related to each other with logical rules which cannot be enforced by a foreign key constraint. These rules can be checked in code, but this may not be the best solution because the data can be changed directly in the table or modified from different parts of code, hence creating possible issues if the code is not kept consistent. So, we will not discuss this approach and concentrate on two others: enforcing referential integrity using check constraints with user defined functions and the second approach - using triggers.

Let's see an example.

Suppose we have a UserList and AdvancedUserList tables in database TestDB. Advanced users have additional attributes, so they are stored in a separate table. In our example a user is considered advanced only when the value of the UserType column in the UserList table equal to 1. In other words, all users that exist in the AdvancedUserList table must exist in the UserList table as well the value of their UserType column in the UserList table must equal 1. We cannot enforce this rule by using a foreign key, because in this way we would not be able to eliminate insertion of users with a type other than 1 into the AdvancedUserList table.

Let's create a test environment to demonstrate:

USE master
GO

CREATE DATABASE TestDB
GO

USE TestDB
GO

CREATE TABLE UserList
(
	UserID INT,
	UserType INT NOT NULL,
	CONSTRAINT PK_UserList_UserID PRIMARY KEY CLUSTERED (UserID)
)
GO

CREATE TABLE AdvancedUserList
(
	UserID INT,
	UserRank INT,
	CONSTRAINT PK_AdvancedUserList_UserID PRIMARY KEY CLUSTERED (UserID)
)
GO

INSERT INTO UserList(UserID, UserType) 
VALUES(1,1),(2,1),(3,2),(4,3),(5,3)
GO

We will use these tables to illustrate our solutions with examples. Let's investigate each solution in detail.

Enforcing SQL Server Referential Integrity using a User Defined Function and Check Constraint

This approach allows us to enforce that the data in the column of the second table will be the subset of filtered data in the first table (users in the second table are the subset of users with UserType = 1 in the first table). First of all we will create a user defined function which checks the existence of the given parameter among UserIDs with UserType = 1 in UserList table. Secondly, we will create a check constraint on the AdvancedUserList table which uses the mentioned function to check if any user in the AdvancedUserList table has a UserType value equal to 1 in the UserList table.

USE TestDB
GO

--Creating function for check constraint
CREATE FUNCTION udfCheckUserID
(
	@pUserID INT
)
RETURNS TINYINT
AS
BEGIN

	IF EXISTS
	(
		SELECT UserID
		FROM AdvancedUserList
		WHERE @pUserID IN (SELECT UserID FROM UserList WHERE UserType=1)
	)
		RETURN 1

	RETURN 0

END
GO

--Creating check constraint which uses the function
ALTER TABLE AdvancedUserList
ADD CONSTRAINT CheckUserID
CHECK(dbo.udfCheckUserID(UserID)=1)

By running the following script we can check how our enforcement works:

USE TestDB
GO

INSERT INTO AdvancedUserList(UserID, UserRank)
VALUES (1,100), (2,100)

INSERT INTO AdvancedUserList(UserID, UserRank)
VALUES (3,100)

SELECT *
FROM AdvancedUserList

We can see that only the first INSERT statement succeeds, because for users with UserID=1 and UserID=2, the UserType value for these two rows is equal to 1 in the UserList table. But for the user with UserID=3, the UserType value is equal to 2, so the check constraint does not allow us to insert this user's data into the AdvancedUserList table:

INSERT statement

As a result, the AdvancedUserList table contains only two rows:

AdvancedUserList

Altering a SQL Server User Defined Function when it is used as a Check Constraint

It is important to mention some essential points of this solution. Firstly, if the function is used by a check constraint, it cannot be altered. Below you see the error message if we try to ALTER this function.

SELECT

To modify the function, we need to drop the check constraint first, alter the function and then recreate the check constraint.

Referential Integrity Not Fully Enforced with User Defined Function Check Constraint

However the most important point of this approach is that it does not fully enforce referential integrity. It does not deny data deletion from the first-parent table, which also exists in the second table:

Use TestDB
GO

--Deleting user from UserList table which exists in AdvancedUserList table
DELETE FROM UserList 
WHERE UserID=2


SELECT *
FROM UserList

SELECT *
FROM AdvancedUserList

The user from the UserList table is successfully deleted despite the fact that it exists in the AdvancedUserList table:

UserList

Moreover we cannot a create check constraint on the UserList table which will deny deletion of records existing in the AdvancedUserList table, because during DELETE statements check constrains are not validated. In case of foreign keys we can define what action should be taken when data is deleted or updated in the parent table. By default it is "NO ACTION" which means when a referenced row is deleted (updated) from the parent table the database engine raises an error and the transaction is rolled back.

In the current example it is possible to use a foreign key and check constraint together: we can create a foreign key on table AdvancedUserList which enforces users in this table to be a subset of users in the UserList table. This will eliminate the deletion of referenced rows from the UserList table and the check constraint ensures insertion of only advanced users into the AdvancedUserList table (only users with UserType=1).

However there could be situations when the creation of a foreign key is not possible. For example, when related columns, unlike our example, have a large varchar data type and the creation of unique index is not possible. It is also a possible case that we may need to implement referential integrity when data in the second table is referred to data from more than one table.

So, generally this approach solves our problem partly. It ensures referential integrity only when data is inserted into the second table. If further modifications of existing referenced rows in the parent table are possible, the referential integrity will be damaged.

Enforcing SQL Server Referential Integrity using Triggers

Now we will use triggers to enforce referential integrity.

In the following code we drop the constraint on the AdvancedUserList table, reinitialize the data and create a trigger on the AdvancedUserList table:

USE TestDB

--Dropping the check constraint
ALTER TABLE AdvancedUserList
DROP CONSTRAINT CheckUserID
GO

--Reinitializing data 
TRUNCATE TABLE AdvancedUserList
TRUNCATE TABLE UserList
GO

INSERT INTO UserList(UserID, UserType) 
VALUES(1,1),(2,1),(3,2),(4,3),(5,3)
GO

--Creating the trigger
CREATE TRIGGER trgCheckUserIDAdvancedUserList
ON AdvancedUserList
AFTER INSERT, UPDATE
AS
BEGIN
	IF EXISTS
	(
		SELECT UserID
		FROM inserted
		WHERE UserID NOT IN (SELECT UserID FROM UserList WHERE UserType=1)
	)
	ROLLBACK TRANSACTION
END
	

This trigger ensures that any insert or update into the AdvancedUserList table will be rolled back if the inserted (updated) UserID doesn't exist in the UserList table where UserType=1:

USE TestDB

INSERT INTO AdvancedUserList(UserID, UserRank)
VALUES (1,100), (2,100)

INSERT INTO AdvancedUserList(UserID, UserRank)
VALUES (3,100)
GO

SELECT *
FROM AdvancedUserList

The result shows that the last insert is rolled back in the trigger, because for user with UserID=3 the UserList table has UserType = 2:

UserID=3

Therefore only two rows are inserted into the AdvancedUserList table:

UserType

To eliminate deleting rows from the UserList table that exist in the AdvancedUserList table we will create a trigger on the UserList table as well:

USE TestDB

CREATE TRIGGER trgCheckUserIDUserList
ON UserList
AFTER DELETE
AS
BEGIN
	IF EXISTS
	(
		SELECT UserID
		FROM deleted
		WHERE UserID IN (SELECT UserID FROM AdvancedUserList)
	)
	ROLLBACK TRANSACTION
END

Let's try to delete rows from the UserList table:

SELECT * 
FROM UserList

SELECT *
FROM AdvancedUserList

DELETE FROM UserList WHERE UserID=5

DELETE FROM UserList WHERE UserID=2
GO

SELECT *
FROM UserList

SELECT *
FROM AdvancedUserList

The row with UserID=5 is successfully deleted, because this user doesn't have data in the AdvancedUserList table. However deletion of row with UserID=2 is rolled back in the trigger, because it exists in the AdvancedUserList table:

trigger

As we can see the row with UserID=5 has been successfully deleted:

referenced data

So, unlike the first approach, using triggers we will not only ensure that data in the column of the second table will be the subset of the data in the corresponding column of the first table, but it also eliminates deletion of referenced data in the first (parent) table.

Conclusion

In conclusion, besides using foreign keys there are other ways to implement referential integrity. Moreover in some cases using foreign keys is impossible, so we need to choose alternatives. Using check constraints with user defined function is one approach, but it does not offer a full solution to ensuring data integrity. Implementing referential integrity using triggers is also a solution and in this way we can enforce full integrity.

Next Steps

Read this related information:



Last Updated: 2016-03-31


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About the author
MSSQLTips author Sergey Gigoyan Sergey Gigoyan is a database professional with more than 10 years of experience, with a focus on database design, development, performance tuning, optimization, high availability, BI and DW design.

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Sunday, July 22, 2018 - 12:23:22 PM - DANIEL SAMIANI Back To Top

Another example can be when a child record should refer to either of two parent tables. For example, a user record can belong to either group 1 or group 2 employees. 

When we use foreign keys there can be only 1 parent for a child and it although in most cases the issue can be bypassed by other design techniques (like creating a third table, which negatively and maybe unnecessarily affects performance), in some cases the limitation causes further problems, and the solution as mentioned in the article can be avoiding foreign keys.

Referential integrity is fine by itself. It's a very good thing that helps maintain data integrity. The issue is with the foreign key constraint in particular that cannot always satisfy the requirement.


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