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.

Practice

1. What does querying by partition key in DynamoDB do?

easy

A. Updates all items with the same partition key value

B. Deletes all items with the same partition key value

C. Fetches all items with the same partition key value

D. Creates a new item with the partition key value

Solution

1. Step 1: Understand partition key role

   The partition key uniquely identifies the partition where items are stored.

2. Step 2: Query by partition key fetches items

   Querying by partition key returns all items that share that key value.

3. Final Answer:

   Fetches all items with the same partition key value -> Option C

4. Quick Check:

   Query by partition key = fetch items [OK]

Hint: Partition key query fetches matching items only [OK]

Common Mistakes:

• Thinking it deletes or updates items
• Confusing partition key with sort key
• Assuming it creates new items

2. Which of the following is the correct syntax to query items by partition key 'UserID' with value '123' in DynamoDB?

easy

A. Table.query(KeyConditionExpression: 'UserID = 123')

B. Table.query(KeyConditionExpression: 'UserID = :val', ExpressionAttributeValues: {':val': '123'})

C. Table.query(FilterExpression: 'UserID = 123')

D. Table.query(KeyConditionExpression: 'UserID == 123')

Solution

1. Step 1: Identify correct query syntax

   DynamoDB requires KeyConditionExpression with placeholders and ExpressionAttributeValues.

2. Step 2: Check each option

   Table.query(KeyConditionExpression: 'UserID = :val', ExpressionAttributeValues: {':val': '123'}) uses correct syntax with placeholder ':val' and value mapping. Others misuse operators or omit placeholders.

3. Final Answer:

   Table.query(KeyConditionExpression: 'UserID = :val', ExpressionAttributeValues: {':val': '123'}) -> Option B

4. Quick Check:

   Use placeholders and ExpressionAttributeValues [OK]

Hint: Use placeholders (:val) and ExpressionAttributeValues [OK]

Common Mistakes:

• Using '==' instead of '=' in KeyConditionExpression
• Not using ExpressionAttributeValues for values
• Using FilterExpression instead of KeyConditionExpression

3. Given a DynamoDB table with items: {UserID: '123', Name: 'Alice'}, {UserID: '123', Name: 'Bob'}, {UserID: '456', Name: 'Carol'}, what will the query by partition key 'UserID' = '123' return?

medium

A. [{UserID: '123', Name: 'Alice'}, {UserID: '123', Name: 'Bob'}]

B. [{UserID: '456', Name: 'Carol'}]

C. [{UserID: '123', Name: 'Alice'}]

D. [] (empty list)

Solution

1. Step 1: Identify items with partition key '123'

   Items with UserID '123' are Alice and Bob.

2. Step 2: Query returns all matching items

   Query returns both Alice and Bob items as they share the partition key.

3. Final Answer:

   [{UserID: '123', Name: 'Alice'}, {UserID: '123', Name: 'Bob'}] -> Option A

4. Quick Check:

   Query by '123' returns Alice and Bob [OK]

Hint: Query returns all items with matching partition key [OK]

Common Mistakes:

• Expecting only one item returned
• Confusing partition key with sort key filtering
• Assuming query returns items with different partition keys

4. You wrote this query to get items with partition key 'OrderID' = '789': Table.query(KeyConditionExpression: 'OrderID = 789'). It returns an error. What is the problem?

medium

A. Partition key name is case-sensitive and should be 'orderid'

B. Using wrong operator '==' instead of '='

C. Query method does not support KeyConditionExpression

D. Missing ExpressionAttributeValues for the value '789'

Solution

1. Step 1: Check query syntax requirements

   DynamoDB requires placeholders and ExpressionAttributeValues for values in KeyConditionExpression.

2. Step 2: Identify missing part

   The query uses 'OrderID = 789' directly without placeholder or ExpressionAttributeValues, causing error.

3. Final Answer:

   Missing ExpressionAttributeValues for the value '789' -> Option D

4. Quick Check:

   Use placeholders and ExpressionAttributeValues [OK]

Hint: Always use placeholders and ExpressionAttributeValues [OK]

Common Mistakes:

• Using raw values instead of placeholders
• Assuming case-insensitivity for partition key
• Using '==' operator instead of '='

5. You want to query a DynamoDB table for all items with partition key 'Category' = 'Books' and sort key 'Price' less than 20. Which approach correctly uses query by partition key with a condition on sort key?

hard

A. Use KeyConditionExpression: 'Category = :cat AND Price < :price' with ExpressionAttributeValues {':cat': 'Books', ':price': 20}

B. Use FilterExpression: 'Category = :cat AND Price < :price' with ExpressionAttributeValues {':cat': 'Books', ':price': 20}

C. Use KeyConditionExpression: 'Category = :cat' and FilterExpression: 'Price < :price' with ExpressionAttributeValues {':cat': 'Books', ':price': 20}

D. Use FilterExpression: 'Category = :cat' and KeyConditionExpression: 'Price < :price' with ExpressionAttributeValues {':cat': 'Books', ':price': 20}

Solution

1. Step 1: Understand query conditions

   KeyConditionExpression must include partition key equality and supports sort key conditions like <. FilterExpression filters results after the query.

2. Step 2: Analyze options

   Use KeyConditionExpression: 'Category = :cat AND Price < :price' with ExpressionAttributeValues {':cat': 'Books', ':price': 20} correctly uses KeyConditionExpression: 'Category = :cat AND Price < :price', which efficiently queries the partition with a sort key range condition. Use KeyConditionExpression: 'Category = :cat' and FilterExpression: 'Price < :price' with ExpressionAttributeValues {':cat': 'Books', ':price': 20} uses FilterExpression for Price (valid but less efficient). B lacks KeyConditionExpression. D omits partition key from KeyConditionExpression.

3. Final Answer:

   Use KeyConditionExpression: 'Category = :cat AND Price < :price' with ExpressionAttributeValues {':cat': 'Books', ':price': 20} -> Option A

4. Quick Check:

   KeyConditionExpression: partition = AND sort < [OK]

Hint: KeyConditionExpression: partition = AND sort condition [OK]

Common Mistakes:

• Using FilterExpression for partition key
• Using FilterExpression instead of KeyConditionExpression for sort key range
• Mixing KeyConditionExpression and FilterExpression incorrectly