Query by partition key in DynamoDB - Mini Project: Build & Apply

Query Items by Partition Key in DynamoDB

📖 Scenario: You are managing a DynamoDB table that stores information about books in a library. Each book has a unique ISBN as the partition key and contains details like Title and Author.You want to retrieve all information about a specific book by querying the table using its ISBN.

🎯 Goal: Build a DynamoDB query that fetches all attributes of a book using its ISBN as the partition key.

📋 What You'll Learn

Create a dictionary called book_item with the exact keys: 'ISBN', 'Title', and 'Author' with given values.

Create a variable called isbn_to_query and set it to the exact string value of the ISBN you want to query.

Write a query expression dictionary called query_params that uses isbn_to_query as the partition key value for the KeyConditionExpression.

Add the final call to dynamodb_client.query() with query_params to complete the query setup.

💡 Why This Matters

🌍 Real World

Querying by partition key is a common operation to retrieve specific items quickly from DynamoDB tables in real-world applications like inventory systems, user profiles, or product catalogs.

💼 Career

Understanding how to query DynamoDB by partition key is essential for backend developers and cloud engineers working with AWS services to build scalable and efficient data-driven applications.

1

Create the book item dictionary

Create a dictionary called book_item with these exact entries: 'ISBN': '978-0132350884', 'Title': 'Clean Code', and 'Author': 'Robert C. Martin'.

DynamoDB

# Create the book_item dictionary with ISBN, Title, and Author
# Your code here

Hint

Use curly braces {} to create a dictionary with the exact keys and values.

2

Set the ISBN to query

Create a variable called isbn_to_query and set it to the string '978-0132350884'.

DynamoDB

book_item = {'ISBN': '978-0132350884', 'Title': 'Clean Code', 'Author': 'Robert C. Martin'}
# Create isbn_to_query variable with the exact ISBN string
# Your code here

Hint

Assign the exact ISBN string to the variable isbn_to_query.

3

Create the query parameters dictionary

Create a dictionary called query_params with the keys TableName set to 'Books', and KeyConditionExpression set to 'ISBN = :isbn'. Also add ExpressionAttributeValues with {':isbn': {'S': isbn_to_query}}.

DynamoDB

book_item = {'ISBN': '978-0132350884', 'Title': 'Clean Code', 'Author': 'Robert C. Martin'}
isbn_to_query = '978-0132350884'
# Create query_params dictionary with TableName, KeyConditionExpression, and ExpressionAttributeValues
# Your code here

Hint

Use the exact keys and values as shown to build the query parameters dictionary.

4

Complete the query call

Add a line that calls dynamodb_client.query() with the argument **query_params to perform the query.

DynamoDB

book_item = {'ISBN': '978-0132350884', 'Title': 'Clean Code', 'Author': 'Robert C. Martin'}
isbn_to_query = '978-0132350884'
query_params = {
    'TableName': 'Books',
    'KeyConditionExpression': 'ISBN = :isbn',
    'ExpressionAttributeValues': {':isbn': {'S': isbn_to_query}}
}
# Call dynamodb_client.query() with query_params
# Your code here

Hint

Use the double asterisk ** to unpack the query_params dictionary as arguments.

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