Composite primary keys in SQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of a query on a table with composite primary key

Consider a table Orders with a composite primary key on (OrderID, ProductID). The table has the following rows:

OrderID | ProductID | Quantity
--------+-----------+---------
1       | 101       | 2
1       | 102       | 1
2       | 101       | 5
2       | 103       | 3

What will be the output of this query?

SELECT OrderID, ProductID FROM Orders WHERE OrderID = 1;

SQL

SELECT OrderID, ProductID FROM Orders WHERE OrderID = 1;

A [{"OrderID":1,"ProductID":101},{"OrderID":1,"ProductID":102}]

B [{"OrderID":1,"ProductID":101}]

C [{"OrderID":1,"ProductID":102},{"OrderID":2,"ProductID":101}]

D []

💡 Hint

Remember that the composite primary key means each combination of OrderID and ProductID is unique, but multiple rows can share the same OrderID.

The query filters rows where OrderID = 1. There are two such rows with ProductID 101 and 102 respectively, so both are returned.

🧠 Conceptual

intermediate

Understanding composite primary keys uniqueness

Which statement best describes the uniqueness enforced by a composite primary key on columns (A, B)?

A Each value in column A must be unique across the table.

B Either column A or column B must have unique values, but not necessarily both.

C Each combination of values in columns A and B must be unique across the table.

D Each value in column B must be unique across the table.

💡 Hint

Think about how composite keys combine columns to enforce uniqueness.

A composite primary key requires that the pair (A, B) is unique for every row, but individual columns can have duplicates.

📝 Syntax

advanced

Correct syntax to define a composite primary key

Which of the following SQL statements correctly defines a composite primary key on columns user_id and role_id in a table UserRoles?

A CREATE TABLE UserRoles (user_id INT PRIMARY KEY, role_id INT PRIMARY KEY);

B CREATE TABLE UserRoles (user_id INT, role_id INT, PRIMARY KEY (user_id, role_id));

C CREATE TABLE UserRoles (user_id INT, role_id INT, PRIMARY KEY user_id, role_id);

D CREATE TABLE UserRoles (user_id INT, role_id INT, PRIMARY KEY user_id AND role_id);

💡 Hint

Look for the correct syntax to declare multiple columns as a single primary key.

Option B correctly uses parentheses to list columns for the composite primary key. Options B, C, and D have syntax errors or misuse keywords.

❓ optimization

advanced

Indexing considerations with composite primary keys

You have a table with a composite primary key on (customer_id, order_id). You frequently query the table filtering only by order_id. What is the best indexing strategy to optimize these queries?

A Create a composite index on <code>(order_id, customer_id)</code>.

B Drop the primary key and create separate primary keys on each column.

C Rely only on the composite primary key index on <code>(customer_id, order_id)</code>.

D Create an additional index on <code>order_id</code> alone.

💡 Hint

Think about how indexes work with leading columns in composite keys.

The composite primary key index is ordered by customer_id first, so queries filtering only by order_id cannot efficiently use it. Creating a separate index on order_id improves performance.

🔧 Debug

expert

Identify the error in composite primary key constraint

Given the following table creation statement, what error will occur?

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

A SyntaxError: Missing parentheses around columns in PRIMARY KEY definition.

B No error; table created successfully with composite primary key.

C TypeError: Columns must be declared as NOT NULL for primary key.

D Runtime error: Duplicate key values not allowed.

💡 Hint

Check the syntax for declaring composite primary keys.

The PRIMARY KEY clause requires parentheses around the column list. Omitting them causes a syntax error.

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