Sort key purpose and usage in DynamoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Purpose of a Sort Key in DynamoDB

What is the main purpose of a sort key in a DynamoDB table?

A To allow multiple items with the same partition key and sort them within that partition

B To uniquely identify each item in the table by itself

C To encrypt the data stored in the table

D To define the read capacity units for the table

💡 Hint

Think about how DynamoDB organizes items when they share the same partition key.

The sort key allows multiple items to share the same partition key but be uniquely identified and sorted within that partition. It helps organize data and enables range queries.

❓ query_result

intermediate

Query Result Using Sort Key Condition

Given a DynamoDB table with partition key 'UserID' and sort key 'Timestamp', which query will return all items for UserID '123' with Timestamp greater than 1000?

DynamoDB

Query(
  TableName='UserActivity',
  KeyConditionExpression='UserID = :uid AND Timestamp > :ts',
  ExpressionAttributeValues={':uid': '123', ':ts': 1000}
)

A Returns all items with UserID '123' and Timestamp greater than 1000

B Returns all items with UserID '123' regardless of Timestamp

C Returns all items with Timestamp greater than 1000 regardless of UserID

D Returns no items because the query syntax is invalid

💡 Hint

Check how the KeyConditionExpression uses both partition and sort keys.

The query filters items by partition key 'UserID' equal to '123' and sort key 'Timestamp' greater than 1000, returning only matching items.

📝 Syntax

advanced

Correct Syntax for Defining Sort Key in Table Creation

Which option shows the correct syntax to define a sort key named 'OrderDate' when creating a DynamoDB table?

DynamoDB

CreateTable(
  TableName='Orders',
  KeySchema=[
    {'AttributeName': 'CustomerID', 'KeyType': 'HASH'},
    {'AttributeName': 'OrderDate', 'KeyType': 'RANGE'}
  ],
  AttributeDefinitions=[
    {'AttributeName': 'CustomerID', 'AttributeType': 'S'},
    {'AttributeName': 'OrderDate', 'AttributeType': 'S'}
  ],
  ProvisionedThroughput={'ReadCapacityUnits': 5, 'WriteCapacityUnits': 5}
)

A KeySchema includes CustomerID as RANGE and OrderDate as HASH; AttributeDefinitions define both as type String

B KeySchema includes OrderDate as HASH and CustomerID as RANGE; AttributeDefinitions define both as type Number

C KeySchema includes only CustomerID as HASH; AttributeDefinitions define OrderDate as type String

D KeySchema includes CustomerID as HASH and OrderDate as RANGE; AttributeDefinitions define both as type String

💡 Hint

Remember HASH is partition key and RANGE is sort key.

The partition key must be defined as 'HASH' and the sort key as 'RANGE' in KeySchema. AttributeDefinitions must match the attribute names and types.

❓ optimization

advanced

Optimizing Queries Using Sort Key

You want to efficiently retrieve all orders for a customer placed within a specific date range. Which approach best uses the sort key to optimize this query?

A Use a Query operation with only the partition key CustomerID and filter dates after fetching

B Use a Scan operation filtering by CustomerID and date range

C Use a Query operation with partition key as CustomerID and a sort key condition between the start and end dates

D Use a GetItem operation with CustomerID and date range as keys

💡 Hint

Consider how Query and Scan differ and how sort key conditions help.

Using Query with partition key and sort key range condition efficiently fetches only relevant items without scanning the whole table.

🔧 Debug

expert

Debugging a Query with Sort Key Condition Error

A developer runs this query on a DynamoDB table with partition key 'UserID' and sort key 'CreatedAt': Query( TableName='Users', KeyConditionExpression='UserID = :uid AND CreatedAt = :date', ExpressionAttributeValues={':uid': 'u123', ':date': '2023-01-01'} ) What error will this query produce?

A ResourceNotFoundException because table 'Users' does not exist

B ValidationException due to missing ExpressionAttributeValue for ':date'

C No error; query returns items with UserID 'u123' and any CreatedAt

D SyntaxError due to incorrect KeyConditionExpression format

💡 Hint

Check if all placeholders in the expression have matching values.

The query references ':date' in the KeyConditionExpression but does not provide a value for it in ExpressionAttributeValues, causing a validation error.