Composite primary keys in SQL - Mini Project: Build & Apply

Creating a Table with Composite Primary Keys

📖 Scenario: You are working on a small library database. Each book can have multiple authors, and each author can write multiple books. To keep track of which author wrote which book, you need a table that connects authors and books.

🎯 Goal: Create a table called BookAuthors that uses a composite primary key made up of BookID and AuthorID. This will ensure that each combination of book and author is unique.

📋 What You'll Learn

Create a table named BookAuthors

Include two columns: BookID and AuthorID, both integers

Set a composite primary key using both BookID and AuthorID

💡 Why This Matters

🌍 Real World

Composite primary keys are used in many-to-many relationships, like linking books and authors, students and courses, or products and orders.

💼 Career

Understanding composite primary keys is important for designing relational databases that accurately model complex relationships and maintain data integrity.

1

Create the BookAuthors table with columns

Write a SQL statement to create a table called BookAuthors with two columns: BookID and AuthorID, both of type INT.

SQL

CREATE TABLE BookAuthors (
    BookID INT,
    AuthorID INT
);
-- Your code here

Hint

Use CREATE TABLE followed by the table name and define the columns with their data types inside parentheses.

2

Add a composite primary key constraint

Modify the BookAuthors table creation statement to add a composite primary key constraint on both BookID and AuthorID.

SQL

CREATE TABLE BookAuthors (
    BookID INT,
    AuthorID INT,
    -- Add composite primary key constraint here
);

Hint

Use PRIMARY KEY (BookID, AuthorID) inside the table definition to set the composite key.

3

Insert sample data into BookAuthors

Write SQL insert statements to add these exact rows into BookAuthors: (1, 101), (1, 102), and (2, 101).

SQL

-- Insert the rows (1, 101), (1, 102), and (2, 101) into BookAuthors
-- Your code here

Hint

Use INSERT INTO BookAuthors (BookID, AuthorID) VALUES (..., ...); for each row.

4

Try inserting a duplicate row to see the primary key in action

Write a SQL insert statement to add the row (1, 101) again into BookAuthors. This should fail because of the composite primary key.

SQL

-- Try to insert the duplicate row (1, 101) into BookAuthors
-- Your code here

Hint

Try inserting the same row again to see how the database prevents duplicates with the composite primary key.

Practice

1. What is a composite primary key in a database table?

easy

A. A primary key that uses only one column to identify rows.

B. A primary key made up of two or more columns combined to uniquely identify a row.

C. A key that allows duplicate values in the table.

D. A foreign key that references multiple tables.

Solution

1. Step 1: Understand primary key basics

   A primary key uniquely identifies each row in a table.

2. Step 2: Define composite primary key

   A composite primary key uses two or more columns together to ensure uniqueness.

3. Final Answer:

   A primary key made up of two or more columns combined to uniquely identify a row. -> Option B

4. Quick Check:

   Composite primary key = multiple columns [OK]

Hint: Composite keys combine columns to ensure unique rows [OK]

Common Mistakes:

• Thinking a primary key can have duplicates
• Confusing composite key with foreign key
• Assuming composite key uses only one column

2. Which of the following is the correct syntax to define a composite primary key on columns order_id and product_id in SQL?

easy

A. PRIMARY KEY (order_id, product_id)

B. PRIMARY KEY order_id, product_id

C. PRIMARY KEY order_id & product_id

D. PRIMARY KEY (order_id + product_id)

Solution

1. Step 1: Recall SQL syntax for composite keys

   Composite keys are defined by listing columns inside parentheses separated by commas.

2. Step 2: Match correct syntax

   PRIMARY KEY (order_id, product_id) uses parentheses and comma correctly: PRIMARY KEY (order_id, product_id).

3. Final Answer:

   PRIMARY KEY (order_id, product_id) -> Option A

4. Quick Check:

   Composite key syntax uses parentheses and commas [OK]

Hint: Use parentheses and commas for composite keys [OK]

Common Mistakes:

• Omitting parentheses around columns
• Using symbols like & or + incorrectly
• Listing columns without commas

3. Given the table OrderDetails with composite primary key (order_id, product_id), what will this query return?

SELECT * FROM OrderDetails WHERE order_id = 101;

medium

A. No rows because both keys must be specified.

B. Only one row with order_id 101 and any product_id.

C. An error because product_id is missing in WHERE clause.

D. All rows where order_id is 101, regardless of product_id.

Solution

1. Step 1: Understand composite key usage in queries

   Composite keys uniquely identify rows, but queries can filter by any column(s).

2. Step 2: Analyze the query filter

   The query filters only by order_id = 101, so it returns all rows with that order_id regardless of product_id.

3. Final Answer:

   All rows where order_id is 101, regardless of product_id. -> Option D

4. Quick Check:

   Filtering by part of composite key returns matching rows [OK]

Hint: Filtering by part of composite key returns matching rows [OK]

Common Mistakes:

• Assuming missing key column causes error
• Thinking only full composite key filters work
• Expecting only one row when multiple match

4. You try to create a table with this SQL:

CREATE TABLE Enrollment (
 student_id INT,
 course_id INT,
 PRIMARY KEY student_id, course_id
);

What is the problem?

medium

A. Missing parentheses around the composite key columns.

B. student_id and course_id cannot be primary keys.

C. PRIMARY KEY must be declared after all columns.

D. Composite keys require UNIQUE keyword instead.

Solution

1. Step 1: Check syntax for composite primary key

   Composite keys require parentheses around the column list in PRIMARY KEY declaration.

2. Step 2: Identify error in given SQL

   The statement uses PRIMARY KEY student_id, course_id without parentheses, causing syntax error.

3. Final Answer:

   Missing parentheses around the composite key columns. -> Option A

4. Quick Check:

   Composite keys need parentheses [OK]

Hint: Always use parentheses for composite primary keys [OK]

Common Mistakes:

• Omitting parentheses in PRIMARY KEY clause
• Confusing primary key with unique constraint
• Placing PRIMARY KEY before column definitions

5. You have a table Attendance with columns student_id, class_date, and session. You want to ensure each student can only have one attendance record per class date and session. Which composite primary key should you define?

hard

A. (class_date, session)

B. (student_id, class_date)

C. (student_id, class_date, session)

D. (student_id, session)

Solution

1. Step 1: Understand uniqueness requirement

   Each student must have only one record per class date and session, so all three columns combined must be unique.

2. Step 2: Choose composite key covering all uniqueness factors

   Composite key must include student_id, class_date, and session to enforce this rule.

3. Final Answer:

   (student_id, class_date, session) -> Option C

4. Quick Check:

   Composite key covers all uniqueness columns [OK]

Hint: Include all columns that define uniqueness in composite key [OK]

Common Mistakes:

• Leaving out session or class_date from key
• Using only two columns when three needed
• Confusing foreign keys with primary keys