FOREIGN KEY constraint in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a foreign key constraint linking the orders table to the customers table.

SQL

CREATE TABLE orders (
  order_id INT PRIMARY KEY,
  customer_id INT,
  CONSTRAINT fk_customer FOREIGN KEY (customer_id) REFERENCES [1](customer_id)
);

Drag options to blanks, or click blank then click option'

Aemployees

Bproducts

Corders

Dcustomers

Attempts:

3 left

💡 Hint

Common Mistakes

Referencing the wrong table name in the foreign key constraint.

Using the same table name as the current table.

The foreign key in orders references the customer_id in the customers table to ensure data integrity.

2fill in blank

medium

Complete the code to add a foreign key constraint to the payments table referencing the orders table.

SQL

ALTER TABLE payments
ADD CONSTRAINT fk_order FOREIGN KEY (order_id) REFERENCES [1](order_id);

Drag options to blanks, or click blank then click option'

Aorders

Bcustomers

Cpayments

Dinvoices

Attempts:

3 left

💡 Hint

Common Mistakes

Referencing the wrong table in the foreign key.

Trying to reference the same table being altered.

The foreign key order_id in payments must reference the order_id in the orders table.

3fill in blank

hard

Fix the error in the foreign key constraint definition by completing the code.

SQL

CREATE TABLE order_items (
  item_id INT PRIMARY KEY,
  order_id INT,
  CONSTRAINT fk_order FOREIGN KEY (order_id) REFERENCES [1](order_id)
);

Drag options to blanks, or click blank then click option'

Aorders

Bcustomers

Citems

Dorder_items

Attempts:

3 left

💡 Hint

Common Mistakes

Referencing the same table in the foreign key constraint.

Referencing an unrelated table.

The foreign key order_id in order_items must reference the order_id in the orders table, not itself or unrelated tables.

4fill in blank

hard

Fill both blanks to create a foreign key constraint with cascading delete from employees to departments.

SQL

CREATE TABLE employees (
  emp_id INT PRIMARY KEY,
  dept_id INT,
  CONSTRAINT fk_dept FOREIGN KEY ([1]) REFERENCES departments([2]) ON DELETE CASCADE
);

Drag options to blanks, or click blank then click option'

Adept_id

Bemp_id

Cdepartment_id

Demployee_id

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping foreign key and referenced key columns.

Using incorrect column names.

The foreign key column in employees is dept_id, which references department_id in departments. The ON DELETE CASCADE ensures related employees are deleted if a department is removed.

5fill in blank

hard

Fill all three blanks to create a foreign key constraint on enrollments referencing students with cascading update and delete.

SQL

CREATE TABLE enrollments (
  enrollment_id INT PRIMARY KEY,
  student_id INT,
  CONSTRAINT fk_student FOREIGN KEY ([1]) REFERENCES [2]([3]) ON UPDATE CASCADE ON DELETE CASCADE
);

Drag options to blanks, or click blank then click option'

Astudent_id

Bstudents

Denrollments

Attempts:

3 left

💡 Hint

Common Mistakes

Referencing the wrong table or column.

Forgetting to match foreign key and referenced column names.

The foreign key student_id in enrollments references student_id in the students table. Cascading update and delete keep data consistent when student records change or are removed.

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