FOREIGN KEY constraint in SQL

Introduction

A FOREIGN KEY constraint links two tables together. It makes sure that the data in one table matches data in another table, keeping information correct and connected.

When you want to connect customer orders to the customers who made them.

When you need to ensure that a product in an order exists in the product list.

When you want to prevent deleting a record that is still used in another table.

When you want to organize data into related tables to avoid repeating information.

Syntax

SQL

CREATE TABLE ChildTable (
  column1 datatype,
  column2 datatype,
  ...,
  FOREIGN KEY (column_name) REFERENCES ParentTable(parent_column)
);

The FOREIGN KEY column in the child table must match the data type of the referenced column in the parent table.

The referenced column in the parent table is usually a PRIMARY KEY or UNIQUE.

Examples

This creates an Orders table where CustomerID must match an existing CustomerID in the Customers table.

SQL

CREATE TABLE Orders (
  OrderID int PRIMARY KEY,
  CustomerID int,
  FOREIGN KEY (CustomerID) REFERENCES Customers(CustomerID)
);

This table links students and courses, ensuring both exist in their respective tables.

SQL

CREATE TABLE Enrollment (
  StudentID int,
  CourseID int,
  FOREIGN KEY (StudentID) REFERENCES Students(StudentID),
  FOREIGN KEY (CourseID) REFERENCES Courses(CourseID)
);

Sample Program

This example creates two tables: Departments and Employees. Employees have a DeptID that must exist in Departments. Then it inserts data and shows employee names with their department names.

SQL

CREATE TABLE Departments (
  DeptID int PRIMARY KEY,
  DeptName varchar(50)
);

CREATE TABLE Employees (
  EmpID int PRIMARY KEY,
  EmpName varchar(50),
  DeptID int,
  FOREIGN KEY (DeptID) REFERENCES Departments(DeptID)
);

INSERT INTO Departments VALUES (1, 'Sales');
INSERT INTO Departments VALUES (2, 'HR');

INSERT INTO Employees VALUES (101, 'Alice', 1);
INSERT INTO Employees VALUES (102, 'Bob', 2);

SELECT EmpName, DeptName FROM Employees
JOIN Departments ON Employees.DeptID = Departments.DeptID;

Important Notes

If you try to insert a value in the child table that does not exist in the parent table, the database will give an error.

Deleting a row in the parent table that is referenced by the child table can be blocked or cause changes depending on the FOREIGN KEY settings.

Summary

FOREIGN KEY connects two tables by matching columns.

It helps keep data accurate and related.

Use it to enforce rules about what data can be entered or deleted.

Practice

1. What is the main purpose of a FOREIGN KEY constraint in a database?

easy

A. To store large amounts of text data efficiently

B. To speed up database queries by creating indexes

C. To link two tables by ensuring values in one table match values in another

D. To automatically backup the database

Solution

1. Step 1: Understand the role of FOREIGN KEY

   A FOREIGN KEY connects columns in two tables to keep data related and consistent.

2. Step 2: Compare options with this role

   Only To link two tables by ensuring values in one table match values in another describes linking tables by matching values, which is the purpose of FOREIGN KEY.

3. Final Answer:

   To link two tables by ensuring values in one table match values in another -> Option C

4. Quick Check:

   FOREIGN KEY links tables = A [OK]

Hint: FOREIGN KEY links tables by matching columns [OK]

Common Mistakes:

• Confusing FOREIGN KEY with indexing
• Thinking FOREIGN KEY stores data
• Assuming FOREIGN KEY backs up data

2. Which of the following is the correct syntax to add a FOREIGN KEY constraint to an existing table Orders referencing Customers(CustomerID)?

easy

A. ALTER TABLE Orders ADD FOREIGN KEY (CustomerID) REFERENCES Customers(CustomerID);

B. ALTER TABLE Orders ADD PRIMARY KEY (CustomerID) REFERENCES Customers(CustomerID);

C. ALTER TABLE Orders ADD FOREIGN KEY CustomerID REFERENCES Customers(CustomerID);

D. ALTER TABLE Orders ADD FOREIGN KEY (CustomerID) TO Customers(CustomerID);

Solution

1. Step 1: Recall correct ALTER TABLE syntax for FOREIGN KEY

   The correct syntax uses: ALTER TABLE table_name ADD FOREIGN KEY (column) REFERENCES other_table(column);

2. Step 2: Check each option

   ALTER TABLE Orders ADD FOREIGN KEY (CustomerID) REFERENCES Customers(CustomerID); matches the correct syntax exactly. Options A, B, and C have syntax errors or wrong keywords.

3. Final Answer:

   ALTER TABLE Orders ADD FOREIGN KEY (CustomerID) REFERENCES Customers(CustomerID); -> Option A

4. Quick Check:

   Correct ALTER TABLE FOREIGN KEY syntax = D [OK]

Hint: Use ADD FOREIGN KEY (col) REFERENCES table(col) syntax [OK]

Common Mistakes:

• Using PRIMARY KEY instead of FOREIGN KEY
• Omitting parentheses around column name
• Using TO instead of REFERENCES keyword

3. Given these tables:
CREATE TABLE Customers (CustomerID INT PRIMARY KEY, Name VARCHAR(50));
CREATE TABLE Orders (OrderID INT PRIMARY KEY, CustomerID INT, FOREIGN KEY (CustomerID) REFERENCES Customers(CustomerID));
What happens if you try to insert INSERT INTO Orders (OrderID, CustomerID) VALUES (1, 999); when there is no customer with CustomerID = 999?

medium

A. The insert fails due to FOREIGN KEY constraint violation

B. The insert succeeds but CustomerID is set to NULL

C. The insert succeeds and adds the order with CustomerID 999

D. The insert succeeds but triggers a warning

Solution

1. Step 1: Understand FOREIGN KEY enforcement

   FOREIGN KEY requires the referenced value to exist in the parent table before inserting.

2. Step 2: Apply to the insert statement

   Since CustomerID 999 does not exist in Customers, the insert violates the FOREIGN KEY rule and fails.

3. Final Answer:

   The insert fails due to FOREIGN KEY constraint violation -> Option A

4. Quick Check:

   Insert with missing parent key = fails [OK]

Hint: Insert fails if referenced key doesn't exist [OK]

Common Mistakes:

• Assuming insert sets foreign key to NULL automatically
• Thinking insert triggers only warnings, not errors
• Believing insert succeeds without parent key

4. You have this table creation:
CREATE TABLE Orders (OrderID INT PRIMARY KEY, CustomerID INT, FOREIGN KEY CustomerID REFERENCES Customers(CustomerID));
What is wrong with this statement?

medium

A. PRIMARY KEY cannot be used with FOREIGN KEY in the same table

B. CustomerID must be declared as PRIMARY KEY

C. REFERENCES keyword is not allowed in FOREIGN KEY constraints

D. FOREIGN KEY must be declared with parentheses around the column name

Solution

1. Step 1: Check FOREIGN KEY syntax

   FOREIGN KEY columns must be enclosed in parentheses, like FOREIGN KEY (CustomerID).

2. Step 2: Identify the error in the statement

   The statement misses parentheses around CustomerID in FOREIGN KEY declaration, causing syntax error.

3. Final Answer:

   FOREIGN KEY must be declared with parentheses around the column name -> Option D

4. Quick Check:

   FOREIGN KEY columns need parentheses [OK]

Hint: Always use parentheses around FOREIGN KEY columns [OK]

Common Mistakes:

• Omitting parentheses around foreign key columns
• Thinking PRIMARY KEY conflicts with FOREIGN KEY
• Misunderstanding REFERENCES usage

5. You want to delete a customer from Customers table who has orders in Orders table. The Orders table has a FOREIGN KEY on CustomerID referencing Customers(CustomerID) with ON DELETE CASCADE. What will happen when you delete that customer?

hard

A. The delete fails because orders exist for that customer

B. The customer is deleted and all their orders are automatically deleted

C. The customer is deleted but orders remain with invalid CustomerID

D. The delete succeeds but sets CustomerID in orders to NULL

Solution

1. Step 1: Understand ON DELETE CASCADE effect

   ON DELETE CASCADE means deleting a parent row also deletes all related child rows automatically.

2. Step 2: Apply to deleting a customer with orders

   Deleting the customer will also delete all orders linked by CustomerID in Orders table.

3. Final Answer:

   The customer is deleted and all their orders are automatically deleted -> Option B

4. Quick Check:

   ON DELETE CASCADE deletes related rows [OK]

Hint: ON DELETE CASCADE removes child rows with parent [OK]

Common Mistakes:

• Assuming delete fails due to existing child rows
• Thinking child rows remain with broken references
• Confusing CASCADE with SET NULL behavior