Foreign key linking mental model in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

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

SQL

ALTER TABLE orders ADD CONSTRAINT fk_customer_id FOREIGN KEY ([1]) REFERENCES customers(id);

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

Acustomer_id

Border_id

Cid

Dcustomer_name

Attempts:

3 left

💡 Hint

Common Mistakes

Using the primary key column id of the orders table instead of the foreign key column.

Using a column that does not exist in the orders table.

The foreign key column in the orders table that links to the customers table is customer_id. This column must match the primary key id in customers.

2fill in blank

medium

Complete the code to select all orders with customer names by joining orders and customers using the foreign key.

SQL

SELECT orders.id, customers.name FROM orders JOIN customers ON orders.[1] = customers.id;

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

Aid

Bname

Ccustomer_id

Dorder_id

Attempts:

3 left

💡 Hint

Common Mistakes

Joining on the wrong columns like orders.id = customers.id.

Using a column that does not exist in orders.

The join condition uses the foreign key customer_id in orders to match the primary key id in customers.

3fill in blank

hard

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

SQL

ALTER TABLE orders ADD CONSTRAINT fk_customer FOREIGN KEY ([1]) REFERENCES customers([2]);

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

Acustomer_id

Border_id

Cid

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Referencing a non-primary key column in customers.

Using the wrong column name in orders.

The foreign key column in orders is customer_id and it references the primary key id in customers.

4fill in blank

hard

Fill both blanks to create a table payments with a foreign key linking to orders.

SQL

CREATE TABLE payments (id INT PRIMARY KEY, order_id INT, FOREIGN KEY ([1]) REFERENCES orders([2]));

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

Aorder_id

Bid

Dpayment_id

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong column name for the foreign key.

Referencing a non-primary key column in orders.

The foreign key column in payments is order_id which references the primary key id in orders.

5fill in blank

hard

Fill all three blanks to select payment ids and customer names by joining payments, orders, and customers using foreign keys.

SQL

SELECT payments.id, customers.name FROM payments JOIN orders ON payments.[1] = orders.[2] JOIN customers ON orders.[3] = customers.id;

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

Aorder_id

Bid

Ccustomer_id

Dpayment_id

Attempts:

3 left

💡 Hint

Common Mistakes

Joining on wrong columns like payments.id or orders.customer_id incorrectly.

Mixing up foreign key and primary key columns.

The joins use the foreign key order_id in payments matching id in orders, and customer_id in orders matching id in customers.

Practice

1. What is the main purpose of a foreign key in a database?

easy

A. To link one table to another and ensure data consistency

B. To store large amounts of text data

C. To speed up database queries

D. To create a backup of the database

Solution

1. Step 1: Understand the role of foreign keys

   A foreign key connects one table to another by referencing a primary key in the related table.

2. Step 2: Identify the purpose of this connection

   This connection helps keep data consistent and organized by preventing invalid data entries.

3. Final Answer:

   To link one table to another and ensure data consistency -> Option A

4. Quick Check:

   Foreign key = link tables + data consistency [OK]

Hint: Foreign keys link tables to keep data correct [OK]

Common Mistakes:

• Thinking foreign keys store data themselves
• Confusing foreign keys with indexes
• Believing foreign keys speed up queries directly

2. Which of the following is the correct syntax to declare a foreign key in SQL?

easy

A. FOREIGN KEY column_name REFERENCES other_table

B. PRIMARY KEY (column_name) REFERENCES other_table(other_column)

C. FOREIGN KEY (column_name) REFERENCES other_table(other_column)

D. KEY FOREIGN (column_name) REFERENCES other_table(other_column)

Solution

1. Step 1: Recall the standard foreign key syntax

   The correct syntax includes the keywords FOREIGN KEY, the column in parentheses, then REFERENCES followed by the referenced table and column in parentheses.

2. Step 2: Compare options to syntax

   FOREIGN KEY (column_name) REFERENCES other_table(other_column) matches the correct syntax exactly. Other options have wrong keyword order or missing parentheses.

3. Final Answer:

   FOREIGN KEY (column_name) REFERENCES other_table(other_column) -> Option C

4. Quick Check:

   FOREIGN KEY + REFERENCES + (table.column) = A [OK]

Hint: FOREIGN KEY (col) REFERENCES table(col) is correct syntax [OK]

Common Mistakes:

• Omitting parentheses around column names
• Swapping PRIMARY KEY with FOREIGN KEY
• Incorrect keyword order

3. Given these tables:
CREATE TABLE Authors (AuthorID INT PRIMARY KEY, Name VARCHAR(50));
CREATE TABLE Books (BookID INT PRIMARY KEY, Title VARCHAR(100), AuthorID INT, FOREIGN KEY (AuthorID) REFERENCES Authors(AuthorID));
What happens if you try to insert INSERT INTO Books (BookID, Title, AuthorID) VALUES (1, 'My Book', 99); when there is no author with AuthorID = 99 in Authors?

medium

A. The insert succeeds but AuthorID is set to NULL

B. The insert fails due to foreign key constraint violation

C. The database creates a new author with AuthorID 99 automatically

D. The insert succeeds and adds the book

Solution

1. Step 1: Understand foreign key constraint behavior

   A foreign key requires that the referenced value exists in the parent table to maintain data integrity.

2. Step 2: Apply this to the insert statement

   Since AuthorID 99 does not exist in Authors, the insert violates the foreign key constraint and fails.

3. Final Answer:

   The insert fails due to foreign key constraint violation -> Option B

4. Quick Check:

   Foreign key requires existing parent row = D [OK]

Hint: Foreign key insert fails if parent key missing [OK]

Common Mistakes:

• Assuming automatic creation of missing parent rows
• Thinking insert will succeed with NULL foreign key
• Ignoring foreign key constraints

4. Consider 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. Foreign key cannot reference Customers table

B. CustomerID should be declared as PRIMARY KEY

C. PRIMARY KEY must be declared after FOREIGN KEY

D. Missing parentheses around the foreign key column name

Solution

1. Step 1: Check foreign key syntax

   The foreign key column name must be enclosed in parentheses after FOREIGN KEY.

2. Step 2: Identify the error in the statement

   The statement uses FOREIGN KEY CustomerID without parentheses, which is invalid syntax.

3. Final Answer:

   Missing parentheses around the foreign key column name -> Option D

4. Quick Check:

   FOREIGN KEY (col) needs parentheses [OK]

Hint: Always use parentheses around foreign key columns [OK]

Common Mistakes:

• Omitting parentheses in FOREIGN KEY declaration
• Misordering PRIMARY and FOREIGN KEY declarations
• Confusing foreign key with primary key requirements

5. You have two tables:
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) ON DELETE SET NULL);
If a department is deleted, what happens to employees linked to that department?

hard

A. Their DeptID is set to NULL automatically

B. The delete is blocked and fails

C. Employees linked to that department are deleted

D. Nothing happens; DeptID remains unchanged

Solution

1. Step 1: Understand ON DELETE SET NULL behavior

   This option means when the referenced row is deleted, the foreign key column in dependent rows is set to NULL.

2. Step 2: Apply to Employees and Departments

   Deleting a department sets DeptID to NULL in Employees who referenced it, keeping employees but removing the link.

3. Final Answer:

   Their DeptID is set to NULL automatically -> Option A

4. Quick Check:

   ON DELETE SET NULL means foreign keys become NULL [OK]

Hint: ON DELETE SET NULL clears foreign keys on delete [OK]

Common Mistakes:

• Assuming delete blocks or cascades employees
• Thinking employees get deleted automatically
• Ignoring ON DELETE action effects