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Monday, May 22, 2023

What is Referential Integrity in Database or SQL - MySQL Example Tutorial

Referential Integrity is a set of constraints applied to foreign keys which prevents entering a row in the child table (where you have the foreign key) for which you don't have any corresponding row in the parent table i.e. entering NULL or invalid foreign keys. Referential Integrity prevents your table from having incorrect or incomplete relationships e.g. If you have two tables Order and Customer where Customer is parent table with primary key customer_id and Order is child table with foreign key customer_id. Since as per business rules you can not have an Order without a Customer and this business rule can be implemented using referential integrity in SQL on a relational database.

Referential Integrity will cause failure on any INSERT or UPDATE SQL statement changing the value of customer_id in the child table If the value of customer_id is not present in the Customer table. 

By the way, What is Referential Integrity in SQL is also an important SQL question similar to finding the second highest salary in SQL or the difference between truncate and delete and should be prepared well before going for any job interview, where knowledge of SQL is one of the requirements.

But, if you are new to the SQL world, it's better to start with a comprehensive SQL course like The Complete SQL Bootcamp course by Jose Portilla on Udemy. That will help you to learn SQL better and quicker, and these kinds of articles will also make more sense once you have some SQL knowledge under your belt.

Referential Integrity example in MySQL tables

What is Refrential Integrity in SQL database with MySQL example
Another example of Referential Integrity is Employee and Department relationship. If we have dept_id as a foreign key in the Employee table then by using referential integrity constraints we can avoid creating  Employee without department or non-existing department. 

In short Referential Integrity makes primary key foreign key relationships viable. Let's first create Employee and Department table with a primary key, foreign key, and referential integrity constraints.

CREATE TABLE Department (dept_id INT NOT NULL,
                         dept_name VARCHAR(256),                      
                         PRIMARY KEY (dept_id)) ENGINE=INNODB;

                       emp_name VARCHAR(256),
                       dept_id INT,
                       FOREIGN KEY (dept_id) REFERENCES Department(dept_id)
                       ON DELETE CASCADE) ENGINE=INNODB;

Above SQL statements will create both Department and Employee tables. dept_id is now a foreign key in the Employee table. 

In this SQL, while creating a foreign key we have specified ON DELETE clause which tells, what needs to do when a record from the parent table is deleted. CASCADE referential action allows to delete or update all matching rows from the child table, after deleting a record in the parent table. This way Referential Integrity preserves data integrity of the relationship.

Let's see How Referential Integrity disallows INSERT and UPDATE for a record in the child table for which there is no matching record in the parent table. To check this Referential Integrity example execute the following MySQL queries :

INSERT INTO Department VALUES (1, "Sales");
INSERT INTO Employee VALUES (101, "Rajeev", 2)

mysql> INSERT INTO Employee VALUES (101, "Rajeev", 2)
    -> ;
ERROR 1452 (23000): Cannot ADD OR UPDATE a child row: a FOREIGN KEY constraint fails (`test`.`employee`, CONSTRAINT `employee_ibfk_1` FOREIGN KEY (`dept_id`) REFERENCES `department` (`dept_id`) ON DELETE CASCADE)

When we inserted first record in Department table it ran fine but when we insert a record in Employee table with dept_id = 2 which is not present in Department i.e. parent table, failed to Referential integrity or foreign key constraint check.

If you modify your query and correct dept_id to 1, query will run fine, as shown below

mysql> INSERT INTO Employee VALUES (101, "Rajeev", 1);
Query OK, 1 row affected (0.05 sec)

Now let's delete our only record from Department table and see if matching records on child table is automatically deleted or not.

mysql> DELETE FROM Department;
Query OK, 1 row affected (0.05 sec)

mysql> SELECT * FROM Employee;
Empty SET (0.00 sec)
You see there is no record in the Employee table because of ON DELETE CASCADE, matching records in the child table are deleted. Similarly, you can use ON UPDATE CASCADE to automatically propagate UPDATE from the parent table to child tables.

Advantage of Referential Integrity in RDBMS and SQL

There are several benefit of Referential Integrity in relational database and maintaining integrity of data among parent and child tables. Here are some of the most noticed advantages of Referential Integrity in SQL:

1) Referential Integrity prevents inserting records with incorrect details in table. Any insert or update operation will fail if it doesn't satisfy referential integrity rule.

2) If a records from parent table is deleted, referential integrity allows to delete all related records from child table using cascade-delete functionality.

3) Similar to second advantage if a record i.e. customer_id of a customer is updated on parent table (Customer table) , Referential Integrity helps to update customer_id in child table (Order) using cascade-update.

That's all on What is referential integrity in the database, SQL, and especially in MySQL. We have seen examples of How referential integrity or foreign key constraint works in MySQL. We have also seen an example of CASCADE DELETE which automatically deletes matching records from the child table.

Other SQL and Database tutorials from Javarevisited Blog

Thanks for reading this article so far. If you like this Referential Integrity tutorial, then please share them with your friends and colleagues. If you have any questions or feedback, then please drop a note.

P. S. - If you are new to SQL and looking for free courses to learn Database and SQL then you will also find this list of free SQL and Database courses useful. The list contains free online courses from Udemy, Coursera, Pluralsight, and other platforms to learn Oracle, MySQL, and SQL Server, and ANSI SQL basics. 


Varron said...

Great examples. I have used foreign keys many times but never use referential integrity checks e.g. updating or deleting records in foreign key table. Your examples shows me how important CASCADE ON UPDATE and CASCADE ON DELETE can be. Would be great if you can share similar examples of Referential Integrity Constraint checks from MSSQL Server.

Anonymous said...

I've one confusion. If we insert in to parent table without adding in to child table at the same time and deleting from child table. That would cause violation of Referential integrity?

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