Beginner

133. FOREIGN KEY

Understand how foreign keys connect records between related tables.

📘 Lesson 133 of 728 ⏱ 9 min 🎯 Beginner

A foreign key is a column or combination of columns in one table that references a key in another table. It helps establish relationships and enforce referential integrity.

Foreign keys allow related business information to be stored in separate tables while maintaining a valid relationship between those records.

Orders can contain CustomerID as a foreign key referencing CustomerID in the Customers table.

SQL 
CREATE TABLE Orders (
    OrderID INT PRIMARY KEY,
    CustomerID INT,
    OrderDate DATE,
    FOREIGN KEY (CustomerID)
        REFERENCES Customers(CustomerID)
);

Foreign keys are widely used in banking, e-commerce, HR, healthcare, finance, and other relational systems.

  • A foreign key creates a relationship between tables.
  • It references a key in another table.
  • It supports referential integrity.
  • It is commonly used with primary keys.
  • Foreign keys are important for JOIN operations.
What is a foreign key?

A foreign key is a column or combination of columns that references a key in another table.

Why are foreign keys important?

They help maintain valid relationships between tables and support referential integrity.

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