Foreign key linking mental model in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a foreign key in a database?

A foreign key is a column or set of columns in one table that refers to the primary key in another table. It creates a link between the two tables.

beginner

Why do we use foreign keys?

Foreign keys help keep data consistent by ensuring that the value in one table matches a value in another table. This prevents errors like referencing a non-existent record.

intermediate

How does a foreign key create a mental model of linking tables?

Think of a foreign key as a bridge connecting two tables. It helps you understand how data in one table relates to data in another, like linking a customer to their orders.

intermediate

What happens if you try to insert a foreign key value that doesn't exist in the referenced table?

The database will reject the insert because it violates the foreign key constraint. This keeps data accurate and linked properly.

intermediate

Can a foreign key be null? What does that mean?

Yes, a foreign key can be null if the relationship is optional. It means the record doesn't currently link to any record in the other table.

What does a foreign key in a table do?

A Stores duplicate data

B Creates a new table

C Deletes data from another table

D Links to a primary key in another table

A foreign key links to a primary key in another table to create a relationship.

What happens if you insert a foreign key value that does not exist in the referenced table?

A The database creates a new record automatically

B The insert is rejected

C The foreign key value is ignored

D The database crashes

The database rejects the insert to maintain data integrity.

Which of these best describes a foreign key?

A A unique identifier for a table

B A column that stores text data

C A link to another table's primary key

D A way to speed up queries

A foreign key links to another table's primary key to connect data.

Can a foreign key column contain NULL values?

A Yes, if the relationship is optional

B No, it must always have a value

C Only if the primary key is NULL

D Only in the primary table

Foreign keys can be NULL to indicate no current link.

What is the main purpose of a foreign key constraint?

A To enforce data consistency between tables

B To speed up data entry

C To allow duplicate records

D To delete related records automatically

Foreign key constraints ensure data stays consistent and linked.

Explain in your own words how a foreign key links two tables in a database.

Think about how one table points to another like a bridge.

Describe what happens if you try to add a record with a foreign key value that does not exist in the referenced table.

Consider the database's role in keeping data accurate.

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