Composite primary keys in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a composite primary key in a database?

A composite primary key is a primary key made up of two or more columns that together uniquely identify each row in a table.

beginner

Why would you use a composite primary key instead of a single column primary key?

You use a composite primary key when no single column can uniquely identify a row, but a combination of columns can.

beginner

How do you define a composite primary key in SQL?

You list multiple columns inside the PRIMARY KEY clause, separated by commas, like: PRIMARY KEY (column1, column2).

intermediate

Can a composite primary key contain NULL values in any of its columns?

No, all columns in a composite primary key must have NOT NULL values to ensure uniqueness.

beginner

Example: What is the composite primary key in this table definition? CREATE TABLE Enrollment ( student_id INT, course_id INT, enrollment_date DATE, PRIMARY KEY (student_id, course_id) );

The composite primary key is the combination of student_id and course_id columns. Together, they uniquely identify each enrollment record.

What does a composite primary key consist of?

A Multiple columns combined to uniquely identify a row

B A single column with multiple values

C A foreign key referencing another table

D A column that allows NULL values

A composite primary key uses multiple columns together to uniquely identify each row.

Which SQL syntax correctly defines a composite primary key?

A PRIMARY KEY column1, column2

B PRIMARY KEY (column1, column2)

C PRIMARY KEY column1 AND column2

D PRIMARY KEY column1 OR column2

The correct syntax uses parentheses around the columns separated by commas.

Can any column in a composite primary key contain NULL values?

A No, all columns must be NOT NULL

B Only one column can have NULL

C It depends on the database

D Yes, NULLs are allowed

All columns in a primary key must be NOT NULL to guarantee uniqueness.

Why might you choose a composite primary key?

A To allow duplicate rows

B To allow NULL values in keys

C To speed up queries

D Because no single column uniquely identifies rows

Composite keys are used when a single column is not enough to uniquely identify rows.

In the table Enrollment(student_id, course_id), which is the best primary key?

A student_id only

B course_id only

C student_id and course_id together

D enrollment_date

The combination of student_id and course_id uniquely identifies each enrollment.

Explain what a composite primary key is and why it is useful.

Think about how two or more columns together can identify a record uniquely.

Describe how to define a composite primary key in SQL with an example.

Remember the parentheses and commas between column names.

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