Composite primary key in DynamoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Querying with Composite Primary Key in DynamoDB

Given a DynamoDB table with a composite primary key consisting of PartitionKey and SortKey, what will be the output of the following query?

Query: Get all items where PartitionKey = 'User#123' and SortKey begins with 'Order#'

DynamoDB

Table: Orders
PartitionKey: User#123
SortKey: Order#001, Order#002, Profile#001

Query: PartitionKey = 'User#123' AND begins_with(SortKey, 'Order#')

A Returns all items with PartitionKey 'User#123' including 'Profile#001'

B Returns items with SortKey 'Order#001' and 'Order#002' only

C Returns no items because begins_with is not valid in DynamoDB

D Returns items with SortKey 'Profile#001' only

💡 Hint

Think about how the sort key condition works with begins_with in DynamoDB queries.

The query filters items by PartitionKey 'User#123' and only includes those where the SortKey starts with 'Order#'. So only 'Order#001' and 'Order#002' are returned.

🧠 Conceptual

intermediate

Understanding Composite Primary Key Components

Which two components make up a composite primary key in DynamoDB?

A Partition Key and Sort Key

B Partition Key and Global Secondary Index

C Hash Key and Range Key

D Primary Key and Secondary Key

💡 Hint

One key decides the partition, the other orders items within that partition.

A composite primary key consists of a Partition Key (also called Hash Key) and a Sort Key (also called Range Key). Together they uniquely identify an item.

📝 Syntax

advanced

Correct Syntax for Querying Composite Primary Key

Which of the following DynamoDB query expressions correctly uses the composite primary key to get all items with PartitionKey 'User#456' and SortKey between 'Order#100' and 'Order#200'?

A KeyConditionExpression: 'PartitionKey = :pk AND SortKey IN (:start, :end)'

B KeyConditionExpression: 'PartitionKey = :pk OR SortKey BETWEEN :start AND :end'

C KeyConditionExpression: 'PartitionKey = :pk AND SortKey BETWEEN :start AND :end'

D KeyConditionExpression: 'PartitionKey = :pk AND SortKey > :start AND SortKey < :end'

💡 Hint

Remember the correct operator to specify a range in DynamoDB queries.

The BETWEEN operator is used in KeyConditionExpression to specify a range for the sort key. AND is required to combine partition key equality and sort key range.

🔧 Debug

advanced

Identify the Error in Composite Key Query

Consider this DynamoDB query snippet:

KeyConditionExpression: 'PartitionKey = :pk AND SortKey = :sk'
ExpressionAttributeValues: { ':pk': 'User#789' }

What error will this cause?

A No error, query runs successfully

B Syntax error: Invalid KeyConditionExpression format

C Query returns no results because ':sk' is undefined

D Runtime error: Missing ExpressionAttributeValue for ':sk'

💡 Hint

Check if all placeholders in the expression have matching values.

The query uses ':sk' in the expression but does not provide a value for it in ExpressionAttributeValues, causing a runtime error.

❓ optimization

expert

Optimizing Access Patterns with Composite Primary Keys

You have a DynamoDB table with composite primary key (UserID, Timestamp). You want to efficiently retrieve the latest 5 records for a user. Which approach is best?

A Query with PartitionKey = UserID and SortKey in descending order, limit 5

B Scan the entire table and filter by UserID, then sort by Timestamp

C Use a Global Secondary Index on Timestamp only and filter by UserID

D Query with PartitionKey = UserID and SortKey in ascending order, limit 5

💡 Hint

Think about how DynamoDB stores items sorted by sort key within a partition.

Querying by PartitionKey and sorting SortKey descending allows efficient retrieval of latest records without scanning the whole table.