Query by partition key in DynamoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Querying items by partition key

Given a DynamoDB table named Orders with OrderID as the partition key, what will be the output of this query if the table contains three orders with OrderID values '1001', '1002', and '1003'?

DynamoDB

const params = {
  TableName: 'Orders',
  KeyConditionExpression: 'OrderID = :id',
  ExpressionAttributeValues: {
    ':id': '1002'
  }
};

// Assume docClient.query(params).promise() is called and returns Items

A [{ OrderID: '1001', Customer: 'Bob', Amount: 100 }]

B [{ OrderID: '1002', Customer: 'Alice', Amount: 250 }]

C []

D SyntaxError: Missing ExpressionAttributeNames

💡 Hint

Remember, querying by partition key returns all items with that key.

The query filters items where OrderID equals '1002'. Only the order with OrderID '1002' is returned.

📝 Syntax

intermediate

Identify the syntax error in DynamoDB query

Which option contains a syntax error in the DynamoDB query to get items by partition key 'UserID' with value 'user123'?

DynamoDB

const params = {
  TableName: 'Users',
  KeyConditionExpression: 'UserID = :uid',
  ExpressionAttributeValues: {
    ':uid': 'user123'
  }
};

A All syntax is correct

B ExpressionAttributeValues key should be ':UserID' not ':uid'

C KeyConditionExpression should be 'UserID == :uid'

D Missing comma after TableName property

💡 Hint

Check the commas between object properties carefully.

The provided code has correct JavaScript syntax with all necessary commas present.

❓ optimization

advanced

Optimizing a query by partition key with a sort key

You have a DynamoDB table Messages with partition key UserID and sort key Timestamp. You want to query all messages for user 'user42' sent after '2023-01-01'. Which query is the most efficient?

A KeyConditionExpression: 'UserID = :uid AND Timestamp > :time', ExpressionAttributeValues: { ':uid': 'user42', ':time': '2023-01-01' }

B KeyConditionExpression: 'UserID = :uid', FilterExpression: 'Timestamp > :time', ExpressionAttributeValues: { ':uid': 'user42', ':time': '2023-01-01' }

C FilterExpression: 'UserID = :uid AND Timestamp > :time', ExpressionAttributeValues: { ':uid': 'user42', ':time': '2023-01-01' }

D Scan with FilterExpression: 'UserID = :uid AND Timestamp > :time', ExpressionAttributeValues: { ':uid': 'user42', ':time': '2023-01-01' }

💡 Hint

Use KeyConditionExpression to filter by partition and sort keys efficiently.

Option A uses KeyConditionExpression with both partition and sort key conditions, which is efficient. Options A and C use FilterExpression which filters after reading data, less efficient. Option A scans the whole table, least efficient.

🔧 Debug

advanced

Debugging a query that returns no results

A developer runs this DynamoDB query to get items with partition key 'ProductID' = 'P123', but gets an empty result. What is the most likely cause?

DynamoDB

const params = {
  TableName: 'Products',
  KeyConditionExpression: 'ProductID = :pid',
  ExpressionAttributeValues: {
    ':pid': 'p123'
  }
};

A TableName is incorrect

B Missing FilterExpression to narrow results

C Partition key value case mismatch: 'P123' vs 'p123'

D KeyConditionExpression syntax error

💡 Hint

DynamoDB partition keys are case sensitive.

DynamoDB keys are case sensitive. Using 'p123' instead of 'P123' causes no matching items.

🧠 Conceptual

expert

Understanding query behavior with composite keys

In a DynamoDB table with partition key CustomerID and sort key OrderDate, which statement is true about querying by partition key only?

A Querying by partition key is not allowed without a sort key condition

B Querying by partition key requires specifying a sort key condition

C Querying by partition key returns only the latest item by OrderDate

D Querying by partition key returns all items for that CustomerID, regardless of OrderDate

💡 Hint

Think about how DynamoDB organizes items with composite keys.

Querying by partition key alone returns all items with that key, regardless of sort key values.