In this Video, I am going to discuss the Cascading Referential Integrity Constraints in SQL Server with examples. Please watch our previous Video where we discussed How to make Primary Key and Foreign key relationships between more than two tables in SQL Server with examples. As part of this Video, we are going to discuss the following pointers in detail.
What is Cascading Referential Integrity Constraint in SQL Server?. The Complete Microsoft SQL Server Developer Course 2022 [Videos].
- What are Cascading Referential Integrity Constraints in SQL Server?
- What are the Actions Performed By SQL Server?
- Example of Cascading Referential Integrity Constraints in SQL Server.
- Understanding Self Referential Integrity Constraint in SQL Server?
What are Cascading Referential Integrity Constraints in SQL Server?
The Cascading Referential Integrity Constraints in SQL Server are the foreign key constraints that tell SQL Server to perform certain actions whenever a user attempts to delete or update a primary key to which an existing foreign keys point.
What are the Actions Performed By SQL Server?
In order to tell the SQL Server what actions to perform whenever a user trying to delete or update a primary key value to which existing foreign key points, we are provided with the following options while working with Cascading Referential Integrity Constraints
If a user tries to delete or update statement(s) that will affect rows in the foreign key table, then those values will be set to NULL when the primary key record is deleted or updated in the Primary key table. The important thing that we need to keep in mind that the foreign key columns affected must allow NULL values.
If a user tries to delete the statement(s) which will affect the rows in the foreign key table, then those rows will be deleted when the primary key record is deleted. Similarly, if an update statement affects rows in the foreign key table, then those rows will be updated with the value from the primary key record after it has been updated.
If a delete or update statement affects rows in a foreign key table, then all rows containing those foreign keys are set to the default value. All foreign key columns in the related table must have default constraints defined on them.
This is the default action that SQL Server performs. This specifies that if an update or deletes statement affects rows in foreign key tables, then the action will be denied and rolled back. An error message will be raised.
Example of Cascading Referential Integrity Constraints in SQL Server:
Let us understand the Cascading Referential Integrity Constraints in SQL Server with one example. In order to understand this, we need two database tables. So lets first create two tables (Person and Gender) and enforce primary and foreign key constraints as shown below.
Create Gender Table
Create Person Table
In the Person table, the GenderID column is the foreign key which is referencing the ID column of the Gender table. The Foreign key constraints can be added graphically using SSMS or using a query as shown below.
For example, consider the above 2 tables (Person and Gender). If we delete the row with ID = 1 from the Gender table then the row with ID = 1 from the Person table becomes an orphan record. We will not be able to tell the Gender for this row. So, the Cascading referential integrity constraint in SQL Server can be used to define the actions that Microsoft SQL Server should take. By default, we get an error and the DELETE or UPDATE statement is rolled back.
If we want to delete or update a record in the parent table (in this case Gender table) when they have a corresponding child record in the child table (in this case Person table) we are provided with a set of rules to perform delete and update operations known as DELETE rules and UPDATE rules as shown in the below image.
Note: The Delete rules and update rules were not imposed on the master table, they are imposed on the child table that too on the foreign key column. Please have a look at the following diagram for the syntax.
Lets delete the existing Person table and again create the person table as shown below
Now insert the following data into the Person table
After making the relationship with cascade rules now we can perform the update and delete operations on parent table reference column data and affected the corresponding child table reference column data also.
— Not Allowed
When we execute the above query it will give us the below error
DELETE FROM Gender WHERE Id = 2 — Allowed
Once we delete the record from the Gender table with Id = 2, it also automatically delete the record(s) from the Person table where GenderId = 2. Fetch the records from the Person table as Select * from Person
Once we update the record from the Gender table with Id = 2 where Gender = "Male", it also automatically updates the record(s) in the Person table. Fetch the records from the Person table and observe.
What is Self Referential Integrity Constraint in SQL Server?
This is the same as the referential integrity we have learned earlier. In earlier cases, we are binding one column of a table with another column of another table whereas in self-referential integrity we bind a column of a table with another column of the same table i.e. both the foreign key and primary key will be present in one table only.
Let us see an example for understanding this concept.
We have the following employee table having the following records.
In the above table, we are binding the column ManagerID (foreign) with another column of the same table i.e. EmpID (Reference key) to verify the value entered into the ManagerID column to be existing under EmpNo column