Query result ordering (ascending, descending) in DynamoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the order of results returned by this DynamoDB query?

Consider a DynamoDB table with a partition key UserID and a sort key Timestamp. You run this query to get all items for UserID = 'user123':

QueryRequest request = new QueryRequest()
    .withTableName("UserActivity")
    .withKeyConditionExpression("UserID = :uid")
    .withExpressionAttributeValues(Map.of(":uid", new AttributeValue().withS("user123")))
    .withScanIndexForward(true);

What order will the items be returned in?

A Items are returned in ascending order by the sort key (Timestamp).

B Items are returned in descending order by the sort key (Timestamp).

C Items are returned in random order.

D Items are returned in ascending order by the partition key (UserID).

💡 Hint

Look at the withScanIndexForward(true) parameter.

Setting ScanIndexForward to true returns results in ascending order by the sort key.

❓ query_result

intermediate

How to get descending order results from a DynamoDB query?

You want to get the latest activity first for a user in a DynamoDB table with UserID as partition key and Timestamp as sort key. Which parameter should you set in the query?

A Set <code>ScanIndexForward</code> to <code>true</code>.

B Set <code>ReturnConsumedCapacity</code> to <code>INDEXES</code>.

C Set <code>ConsistentRead</code> to <code>true</code>.

D Set <code>ScanIndexForward</code> to <code>false</code>.

💡 Hint

Think about reversing the order of the sort key.

Setting ScanIndexForward to false returns results in descending order by the sort key.

🧠 Conceptual

advanced

Why can't you order DynamoDB query results by attributes other than the sort key?

DynamoDB queries allow ordering results by the sort key only. Why is it not possible to order by other attributes?

A Because DynamoDB stores data sorted only by partition key and sort key, so ordering by other attributes would require scanning all data.

B Because DynamoDB automatically orders by all attributes internally, so specifying others is redundant.

C Because DynamoDB does not support any ordering of query results.

D Because ordering by other attributes is only possible with a Scan operation.

💡 Hint

Think about how DynamoDB stores and indexes data physically.

DynamoDB stores items sorted by partition key and sort key only. Ordering by other attributes would require scanning all items, which is inefficient and not supported in Query.

📝 Syntax

advanced

Identify the syntax error in this DynamoDB query for descending order

Which option contains a syntax error in setting the query to return results in descending order?

QueryRequest request = new QueryRequest()
    .withTableName("UserActivity")
    .withKeyConditionExpression("UserID = :uid")
    .withExpressionAttributeValues(Map.of(":uid", new AttributeValue().withS("user123")))
    .withScanIndexForward(???);

A withScanIndexForward(Boolean.FALSE)

B withScanIndexForward(false)

C withScanIndexForward("false")

D withScanIndexForward(0)

💡 Hint

Check the expected data type for withScanIndexForward.

The withScanIndexForward method expects a boolean value, not a string.

❓ optimization

expert

How to efficiently get the last 5 items in descending order from a DynamoDB table?

You want to get the 5 most recent activities for a user from a DynamoDB table with UserID as partition key and Timestamp as sort key. Which approach is best?

A Query with <code>ScanIndexForward(true)</code> and <code>Limit(5)</code> to get first 5 items in ascending order.

B Query with <code>ScanIndexForward(false)</code> and <code>Limit(5)</code> to get last 5 items in descending order.

C Use a Global Secondary Index on Timestamp and scan it to get last 5 items.

D Scan the whole table and sort results in application code to get last 5 items.

💡 Hint

Think about how to limit data read and get results in desired order.

Querying with ScanIndexForward set to false returns items in descending order by sort key. Using Limit(5) restricts results to 5 items, making it efficient.