One-to-many relationship patterns in DynamoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Querying all orders for a single customer

You have a DynamoDB table named Orders with a partition key CustomerID and a sort key OrderID. Which query will return all orders for customer with ID 123?

A Query with KeyConditionExpression: OrderID = '123'

B Scan with FilterExpression: CustomerID = '123'

C Scan with FilterExpression: OrderID = '123'

D Query with KeyConditionExpression: CustomerID = '123'

💡 Hint

Remember, queries use the partition key to efficiently find items.

Querying with CustomerID = '123' uses the partition key to get all orders for that customer efficiently. Scans are less efficient and filtering on sort key alone won't work.

❓ schema

intermediate

Designing a one-to-many pattern with composite keys

You want to store blog posts and their comments in one DynamoDB table. Each post has many comments. Which key design best supports efficient retrieval of all comments for a post?

A Partition key: PostID, Sort key: CommentID

B Partition key: CommentID, Sort key: PostID

C Partition key: PostID, Sort key: PostID

D Partition key: CommentID only

💡 Hint

Think about grouping comments under the post they belong to.

Using PostID as partition key groups all comments for that post together. CommentID as sort key uniquely identifies each comment.

❓ optimization

advanced

Optimizing queries for recent comments retrieval

You want to get the 5 most recent comments for a post. Comments have a Timestamp attribute. How should you design the sort key to optimize this query?

A Sort key: Timestamp (descending order)

B Sort key: Timestamp (ascending order)

C Sort key: CommentID

D Sort key: PostID

💡 Hint

DynamoDB sort keys are stored in ascending order by default.

Using Timestamp as sort key in ascending order allows querying with ScanIndexForward=false to get recent comments first.

🔧 Debug

advanced

Why does this query return no results?

You run this DynamoDB query to get comments for post p1:

KeyConditionExpression: "PostID = :pid AND CommentID > :cid"
ExpressionAttributeValues: {":pid": "p1", ":cid": "c0"}

But it returns no items, even though comments exist. What is the likely cause?

A ExpressionAttributeValues syntax is wrong

B PostID value is incorrect

C CommentID is not the sort key, so condition on it fails

D KeyConditionExpression cannot use > operator

💡 Hint

KeyConditionExpression conditions apply only to partition and sort keys.

Conditions on attributes that are not partition or sort keys in KeyConditionExpression cause no results. CommentID must be the sort key to use > operator.

🧠 Conceptual

expert

Modeling many comments per post with single table design

You want to store posts and their comments in one DynamoDB table using a single table design. Which pattern correctly models the one-to-many relationship?

A Use partition key as PostID, sort key as 'POST#' for posts and 'COMMENT#CommentID' for comments

B Use partition key as CommentID, sort key as PostID

C Use partition key as PostID only for posts and separate table for comments

D Use partition key as CommentID, sort key as 'POST#PostID'

💡 Hint

Single table design uses composite sort keys to distinguish item types.

Using PostID as partition key groups all items for a post. Sort key prefixes like 'POST#' and 'COMMENT#' distinguish posts from comments and allow querying all related items.