SQL FOREIGN KEY: Understanding and Implementing in Relational Databases

SQL FOREIGN KEY

A FOREIGN KEY is a column in one table that points to the primary key of another table. It establishes relationships between tables and enforces referential integrity ÔÇö preventing invalid or orphaned data.

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Example: Employees and Salaries

The Salaries.Emp_Id column references Employees.Emp_Id. Salaries cannot have an Emp_Id that does not exist in Employees.

Defining a FOREIGN KEY on CREATE TABLE

-- MySQL
CREATE TABLE Salaries (
  Sal_Id INT NOT NULL PRIMARY KEY,
  Amount INT NOT NULL,
  Emp_Id INT,
  FOREIGN KEY (Emp_Id) REFERENCES Employees(Emp_Id)
);

-- SQL Server / Oracle
CREATE TABLE Salaries (
  Sal_Id INT NOT NULL PRIMARY KEY,
  Amount INT NOT NULL,
  Emp_Id INT FOREIGN KEY REFERENCES Employees(Emp_Id)
);

Adding a FOREIGN KEY with ALTER TABLE

ALTER TABLE Salaries
ADD CONSTRAINT fk_EmpSalaries
FOREIGN KEY (Emp_Id) REFERENCES Employees(Emp_Id);

Dropping a FOREIGN KEY

-- MySQL
ALTER TABLE Salaries DROP FOREIGN KEY fk_EmpSalaries;

-- SQL Server / Oracle
ALTER TABLE Salaries DROP CONSTRAINT fk_EmpSalaries;

Cascade Actions

Control what happens when the parent row changes:

FOREIGN KEY (Emp_Id) REFERENCES Employees(Emp_Id)
ON DELETE CASCADE
ON UPDATE CASCADE;

• CASCADE: Apply the change to child rows automatically.
• SET NULL: Set child foreign key to NULL.
• RESTRICT / NO ACTION: Block the change if children exist.

PRIMARY KEY vs FOREIGN KEY

• NULL: Primary Key ÔÇö No; Foreign Key ÔÇö Yes.
• Uniqueness: Primary always unique; Foreign can repeat.
• Quantity: One Primary Key per table; many Foreign Keys allowed.

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Conclusion

Foreign keys keep your data consistent and your relationships rock-solid. Use them everywhere you have parent-child data and prevent orphaned records forever. For more SQL tutorials, stay tuned to .

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