Key-value and document store model in DynamoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Query Result: Retrieving an Item by Primary Key

You have a DynamoDB table named Users with UserID as the primary key. Which query will return the item with UserID equal to 123?

A GetItem with Key = {"UserID": {"N": "123"}}

B Query with KeyConditionExpression = "UserID = :id" and ExpressionAttributeValues = {":id": {"N": "123"}}

C Scan with FilterExpression = "UserID = :id" and ExpressionAttributeValues = {":id": {"S": "123"}}

D UpdateItem with Key = {"UserID": {"N": "123"}}

💡 Hint

Use the simplest method to get an item by its primary key without scanning the whole table.

GetItem is the most efficient way to retrieve a single item by its primary key. Query requires a partition key and is used for composite keys, but GetItem is simpler here. Scan reads the whole table and is inefficient. UpdateItem modifies data, not retrieves.

🧠 Conceptual

intermediate

Understanding Partition Keys in DynamoDB

Which statement best describes the role of a partition key in a DynamoDB table?

A It uniquely identifies each item and determines the partition where the item is stored.

B It is used only for sorting items within a partition.

C It is optional and only used for indexing secondary attributes.

D It stores the actual data values of the item.

💡 Hint

Think about how DynamoDB organizes data physically.

The partition key uniquely identifies each item and decides which partition stores the item. It is mandatory and essential for fast lookups. Sorting happens with a sort key, not the partition key.

📝 Syntax

advanced

Correct Syntax for Adding a New Attribute to an Item

You want to add a new attribute Age with value 30 to an existing item with UserID = 101 in DynamoDB. Which UpdateItem syntax is correct?

A UpdateItem with Key={"UserID": {"N": "101"}}, UpdateExpression="SET Age = 30"

B UpdateItem with Key={"UserID": {"N": "101"}}, UpdateExpression="ADD Age :val", ExpressionAttributeValues={":val": {"N": "30"}}

C UpdateItem with Key={"UserID": {"N": "101"}}, UpdateExpression="SET Age = :val", ExpressionAttributeValues={":val": {"N": "30"}}

D UpdateItem with Key={"UserID": {"N": "101"}}, UpdateExpression="SET Age :val", ExpressionAttributeValues={":val": {"N": "30"}}

💡 Hint

Remember the syntax for UpdateExpression requires an equals sign and placeholders for values.

The correct syntax uses SET Age = :val with a placeholder :val defined in ExpressionAttributeValues. Option C uses ADD which is for numeric increments, not setting values. Option C misses the equals sign. Option C uses a literal value without a placeholder, which is invalid.

❓ optimization

advanced

Optimizing Queries with Secondary Indexes

You have a DynamoDB table with UserID as the partition key and OrderDate as a sort key. You want to efficiently query all orders placed on a specific OrderDate regardless of UserID. What is the best approach?

A Query the table using <code>UserID</code> and filter by <code>OrderDate</code>.

B Use Scan operation with a filter on <code>OrderDate</code>.

C Create a Local Secondary Index (LSI) with <code>OrderDate</code> as the sort key and query it.

D Create a Global Secondary Index (GSI) with <code>OrderDate</code> as the partition key and query it.

💡 Hint

Think about how to query by an attribute that is not the primary key.

A Global Secondary Index allows you to query the table using a different partition key, here OrderDate. Local Secondary Indexes share the same partition key as the base table, so they can't be used to query by OrderDate alone. Scan reads the whole table and is inefficient. Querying by UserID won't help if you want all orders on a date regardless of user.

🔧 Debug

expert

Debugging a Conditional Write Failure

You try to update an item in DynamoDB with a ConditionExpression to only update if Status is "Pending". The update fails with a ConditionalCheckFailedException. What is the most likely cause?

A The <code>Status</code> attribute is misspelled in the condition expression.

B The item does not exist, so the condition cannot be evaluated.

C The <code>ConditionExpression</code> syntax is invalid and causes the failure.

D The update operation does not support conditional expressions.

💡 Hint

Think about what happens if the item you want to update is missing.

If the item does not exist, the condition Status = "Pending" cannot be true, so the conditional update fails with ConditionalCheckFailedException. Misspelling the attribute would cause a different error. The syntax error would cause a syntax exception. Update supports conditional expressions.