Inserting Values into Two Tables with Foreign Key Relationship: A Step-by-Step Guide

Inserting Values into Two Tables with Foreign Key Relationship

When working with databases, it’s common to have multiple tables that are related to each other through foreign keys. In this article, we’ll explore how to insert values into two tables, EMPLOYEE and DEPARTMENT, which have a foreign key relationship.

Understanding the Database Schema

Let’s take a closer look at the database schema:

CREATE TABLE EMPLOYEE (
  Ssn VARCHAR(11) PRIMARY KEY,
  Name VARCHAR(50),
  Dno INTEGER FOREIGN KEY REFERENCES DEPARTMENT(Dnumber)
);

CREATE TABLE DEPARTMENT (
  Dnumber INTEGER PRIMARY KEY,
  Dname VARCHAR(100),
  Mgr_ssn INTEGER FOREIGN KEY REFERENCES EMPLOYEE(Ssn)
);

In this schema, the EMPLOYEE table has a foreign key Dno that references the Dnumber column in the DEPARTMENT table. Similarly, the DEPARTMENT table has a foreign key Mgr_ssn that references the Ssn column in the EMPLOYEE table.

The Challenge

The question asks how to insert values into both tables with the foreign key relationship intact. We’ll explore this in detail below.

Inserting Department First: A Null-Able Manager

One common approach is to insert the department first, with a null-able manager. This way, we can avoid any issues that might arise from trying to insert an employee before the department.

INSERT INTO DEPARTMENT (Dnumber, Dname, Mgr_ssn)
VALUES (5, 'Research', NULL);

This inserts a new row into the DEPARTMENT table with Dnumber = 5, Dname = ‘Research’, and Mgr_ssn = NULL.

Inserting Employee: The Hard Part

Now that we have inserted the department, it’s time to insert an employee. However, because of the foreign key relationship, we can’t just insert any value into the Dno column; we need to make sure that the corresponding department exists with a non-null manager.

INSERT INTO EMPLOYEE (Ssn, Name, Dno)
VALUES ('333445555', 'John', 5);

In this example, we’re inserting an employee with Ssn = ‘333445555’ and Name = ‘John’. The key is that the Dno column references the Dnumber in the DEPARTMENT table.

Updating Department Manager

After inserting the employee, we need to update the department manager. We can do this using an UPDATE statement.

UPDATE DEPARTMENT
SET Mgr_ssn = '333445555'
WHERE Dnumber = 5;

This updates the department with Dnumber = 5, setting its manager’s SSN to ‘333445555’.

Using Surrogate Keys Instead

As a best practice, we should use surrogate keys instead of natural keys like SSNs. Surrogate keys are artificial keys that are generated for each row in the table.

CREATE TABLE EMPLOYEE (
  EmployeeID INTEGER PRIMARY KEY,
  Ssn VARCHAR(11),
  Name VARCHAR(50),
  Dno INTEGER FOREIGN KEY REFERENCES DEPARTMENT(Dnumber)
);

CREATE TABLE DEPARTMENT (
  DepartmentID INTEGER PRIMARY KEY,
  Dnumber INTEGER,
  Dname VARCHAR(100),
  Mgr_ssn INTEGER FOREIGN KEY REFERENCES EMPLOYEE(EmployeeID)
);

In this revised schema, we’ve added EmployeeID and DepartmentID columns as surrogate keys. We can then use these IDs in the foreign key relationships.

Conclusion

Inserting values into two tables with a foreign key relationship requires careful planning and execution. By understanding the database schema, we can insert data in a way that maintains the integrity of the relationship between the tables. Whether using null-able managers or surrogate keys, there are several approaches to consider when working with foreign key relationships.

Additional Considerations

When designing your database schema, keep in mind the following best practices:

Use natural keys only for primary keys, and use surrogate keys for foreign keys.
Make sure foreign keys reference existing data in the referenced table.
Avoid using SSNs or other personal identifiable information as primary keys or foreign keys.

By following these guidelines and understanding the complexities of foreign key relationships, you can build robust and maintainable databases that support your applications’ needs.

Last modified on 2023-10-29