SQL Delete Cascade
In this post, we will look at SQL DELETE CASCADE and UPDATE CASCADE rules in SQL Server foreign keys using multiple examples.
DELETE CASCADE: When we create a foreign key using delete cascade, it deletes the referencing rows in the child table when the referenced row in the parent table with a primary key is deleted.
This means that if a Parent row is deleted, no orphan row in the Child table will be left. All of the children`s rows that refer to the parent’s rows should also be deleted (removed). If any of these children have grandchildren (through another foreign key in another table) and ON DELETE CASCADE is defined, they must be deleted as well.
Below is a SQL script that creates a parent table (Departments), a child table (Employees), and a foreign key to the child table without using the SQL delete cascade rule.
-- Create Departments table Create table Departments ( Id int primary key identity, [DeptName] nvarchar(50) ) Go -- Create Employees table Create table Employees ( Id int primary key identity, [Name] nvarchar(50), [City] nvarchar(50), DeptId int constraint FK_Key foreign key references Departments(Id) ) Go -- Insert records into Departments table Insert into Departments values ('IT') Insert into Departments values ('Sales') Go -- Insert records into Employees table Insert into Employees values ('Shekh Ali','New Delhi', 1), ('Mark Adam','New York', 1), ('Julie Folden','Paris', 2), ('Amit Kumar','Mumbai', 2), ('Priyanka','Noida', 2); -- Select all the records from Departments and Employees table. SELECT * FROM Departments; SELECT * FROM Employees;
After running the above SQL script, the following
Employees tables will be created in the database.
Delete related rows from the parent-child tables in SQL
When we delete a row from the Departments table, all the related rows from the Employees table must also be deleted.
For example, if we delete the IT department row from the Departments (Parent) table, we also want all the employees of the IT department to be deleted from the Employees (Child) table as well.
Here, the DeptId column in the Employees table is a foreign key that is referencing the Id (Primary key) column in the Departments table.
Let’s try to delete the IT department row from the Department’s table by running the following SQL script.
-- Delete IT department DELETE FROM Departments WHERE Id = 1;
If we try to delete a row from the Departments table which has related rows in the Employees table, by default, we get the following reference constraints error message:The DELETE statement conflicted with the REFERENCE constraint “FK_Key”. The conflict occurred in database “TestDB”, table “dbo.Employees”, column ‘DeptId’.
. The statement has been terminated.
Example: SQL Delete Cascade
To apply the SQL
on delete cascade rule, first remove the current foreign key constraint from the Employees (Child) table. Here is a SQL script that will drop the foreign key constraint from the child table.
-- Drop foreign key from the Employees table ALTER TABLE Employees DROP CONSTRAINT FK_Key;
Now let’s recreate the foreign key constraint with cascading delete rule in the Employees table.
-- Create foreign key constraint with delete cascade ALTER TABLE Employees ADD CONSTRAINT FK_Dept_Emp_Cascade_Delete FOREIGN KEY (DeptId) REFERENCES Departments(Id) ON DELETE CASCADE;
Now when we delete a row from the Departments table, all the related rows from the Employees table will also get deleted.
-- Delete IT department DELETE FROM Departments WHERE Id = 1; -- Select all the records from Departments and Employees table. SELECT * FROM Departments; SELECT * FROM Employees;
Once we run the above SQL query, the IT department will get deleted from the Departments table, and also all the referenced rows will be deleted from the Employees table.
In the following image, we can see that the IT department is no more available in the Departments table and referenced rows from the Employees table will be cleaned up automatically.
Row deletion actions in the parent table
When you delete one or more rows from the parent table, you have the option of performing one of the following actions:
|ON DELETE NO ACTION:||SQL Server raises an error and rolls back the delete action on the record in the parent table.|
|ON DELETE CASCADE:||When a row in the parent table is deleted, SQL Server deletes the corresponding row in the child table.|
|ON DELETE SET NULL:||SQL Server sets the rows in the child table to NULL when the corresponding rows in the parent table are deleted. The foreign key columns must be nullable in order for this action to be performed.|
|ON DELETE SET DEFAULT:||When the rows in the parent table are deleted, SQL Server update the rows in the child table to their default values. This process can only be carried out if the foreign key columns contain default definitions.|
If you don’t explicitly specify an action, SQL Server employs
ON DELETE NO ACTION by default.
On Update Cascade in SQL Server
UPDATE CASCADE: When we create a foreign key using
UPDATE CASCADE, the referencing rows in the child table are updated whenever the referred row in the parent table with a primary key is updated.
ALTER TABLE [dbo].[tblChild] ADD CONSTRAINT FK_tblChild FOREIGN KEY([ID]) REFERENCES [dbo].[tblParent]([ID]) ON UPDATE CASCADE
Q: What is the purpose of cascade in SQL?
In SQL, CASCADE is used in conjunction with ON DELETE or ON UPDATE. It means that when the parent row is deleted or updated, the child row is also deleted or updated simultaneously.
Q: How does delete cascade work in SQL?
Use the ON DELETE CASCADE option to specify whether to delete child table rows when matching rows in the parent table are deleted. If you don’t define cascading deletes, the database server prevents you from deleting data in a table when the table is referenced by other tables.
In this article, we learned about Delete Cascade in SQL Server with examples. I hope you enjoyed and learned something from this article. if you have any suggestions, questions, or comments, please share them with us.