What is a FOREIGN KEY and how does it ensure data integrity?

A FOREIGN KEY is a constraint that establishes a relationship between a column or set of columns in one table and a column or columns in another table (typically a primary key).

It ensures referential integrity by guaranteeing that values in the foreign key column correspond to existing values in the related table.

How data integrity is ensured:

• Prevents insertion of invalid data It is impossible to insert a value into the foreign key column if that value does not exist in the related table.

• Prevents deletion of related records It is impossible to delete a record from the parent table if records in the child table reference it, unless additional actions are taken.

Example of a foreign key:

MySQL 8.1
CREATE TABLE Employees (
    EmployeeID INT PRIMARY KEY,
    Name VARCHAR(100),
    DepartmentID INT,
    FOREIGN KEY (DepartmentID) REFERENCES Departments(DepartmentID)
);