Scan with filter expressions in DynamoDB - Mini Project: Build & Apply

Scan with filter expressions

📖 Scenario: You are managing a DynamoDB table that stores information about books in a library. Each book has attributes like Title, Author, and Year. You want to find books published after the year 2000.

🎯 Goal: Build a DynamoDB scan operation with a filter expression to retrieve only books published after the year 2000.

📋 What You'll Learn

Create a dictionary called table_data with three books, each having Title, Author, and Year attributes.

Create a variable called year_threshold and set it to 2000.

Write a scan operation using a filter expression that selects books with Year greater than year_threshold.

Add the final step to execute the scan and store the filtered results in a variable called filtered_books.

💡 Why This Matters

🌍 Real World

Filtering data in DynamoDB tables is common when you want to retrieve only relevant items without fetching the entire table.

💼 Career

Understanding scan operations with filter expressions is essential for backend developers and database administrators working with AWS DynamoDB.

1

Create the initial table data

Create a dictionary called table_data with these exact entries: {'BookID1': {'Title': 'The Hobbit', 'Author': 'J.R.R. Tolkien', 'Year': 1937}, 'BookID2': {'Title': 'Harry Potter and the Goblet of Fire', 'Author': 'J.K. Rowling', 'Year': 2000}, 'BookID3': {'Title': 'The Da Vinci Code', 'Author': 'Dan Brown', 'Year': 2003}}

DynamoDB

# Create the table_data dictionary with the exact book entries
# Your code here

Hint

Use a dictionary with keys as book IDs and values as dictionaries of book attributes.

2

Set the year threshold

Create a variable called year_threshold and set it to 2000.

DynamoDB

table_data = {
    'BookID1': {'Title': 'The Hobbit', 'Author': 'J.R.R. Tolkien', 'Year': 1937},
    'BookID2': {'Title': 'Harry Potter and the Goblet of Fire', 'Author': 'J.K. Rowling', 'Year': 2000},
    'BookID3': {'Title': 'The Da Vinci Code', 'Author': 'Dan Brown', 'Year': 2003}
}
# Create the year_threshold variable and set it to 2000
# Your code here

Hint

Just assign the number 2000 to the variable year_threshold.

3

Write the scan with filter expression

Write a scan operation using a filter expression that selects books with Year greater than year_threshold. Use a variable called filtered_books to store the filtered results. Use a dictionary comprehension to filter table_data where the book's Year is greater than year_threshold.

DynamoDB

filtered_books = {book_id: info for book_id, info in table_data.items() if info['Year'] > year_threshold}
# Finalize the scan operation

Hint

Use a dictionary comprehension with for book_id, info in table_data.items() and filter with if info['Year'] > year_threshold.

4

Complete the scan operation

Add the final step to execute the scan and store the filtered results in a variable called filtered_books. Ensure the code includes the full dictionary comprehension filtering table_data by Year greater than year_threshold.

DynamoDB

filtered_books = {book_id: info for book_id, info in table_data.items() if info['Year'] > year_threshold}

Hint

The final step is to have the filtered_books variable assigned with the filtered dictionary comprehension.

Practice

1. What does a Scan operation with a filter expression do in DynamoDB?

easy

A. It deletes items that do not match the filter condition.

B. It reads only the items that match the filter condition directly.

C. It updates items that match the filter condition.

D. It reads all items but returns only those matching the filter condition.

Solution

1. Step 1: Understand Scan operation

   A Scan reads every item in the table regardless of any condition.

2. Step 2: Apply filter expression effect

   The filter expression is applied after reading all items, so only matching items are returned.

3. Final Answer:

   It reads all items but returns only those matching the filter condition. -> Option D

4. Quick Check:

   Scan + filter = read all, return filtered [OK]

Hint: Scan reads all, filter returns matching items only [OK]

Common Mistakes:

• Thinking Scan reads only filtered items
• Confusing Scan with Query
• Assuming filter modifies data

2. Which of the following is the correct syntax to use a filter expression in a DynamoDB Scan operation?

easy

A. scan(TableName='MyTable', Filter='Name = "John"')

B. scan(TableName='MyTable', ConditionExpression='Name = "John"')

C. scan(TableName='MyTable', FilterExpression='attribute_exists(Name)')

D. scan(TableName='MyTable', FilterCondition='Name = "John"')

Solution

1. Step 1: Identify correct parameter name

   The correct parameter for filtering in Scan is FilterExpression.

2. Step 2: Check syntax correctness

   scan(TableName='MyTable', FilterExpression='attribute_exists(Name)') uses FilterExpression with a valid condition attribute_exists(Name).

3. Final Answer:

   scan(TableName='MyTable', FilterExpression='attribute_exists(Name)') -> Option C

4. Quick Check:

   FilterExpression is correct parameter [OK]

Hint: Use FilterExpression parameter for scan filters [OK]

Common Mistakes:

• Using ConditionExpression instead of FilterExpression
• Using Filter or FilterCondition which are invalid
• Incorrect syntax for filter condition

3. Given a DynamoDB table with items: [{"Name": "Alice", "Age": 30}, {"Name": "Bob", "Age": 25}, {"Name": "Carol", "Age": 35}], what will be the result of a Scan with filter expression Age > 30?

medium

A. [{"Name": "Alice", "Age": 30}]

B. [{"Name": "Carol", "Age": 35}]

C. [{"Name": "Bob", "Age": 25}]

D. [{"Name": "Alice", "Age": 30}, {"Name": "Carol", "Age": 35}]

Solution

1. Step 1: Understand filter condition

   The filter expression Age > 30 means only items with Age greater than 30 are returned.

2. Step 2: Check each item against condition

   Alice has Age 30 (not greater), Bob 25 (not greater), Carol 35 (greater). Only Carol matches.

3. Final Answer:

   [{"Name": "Carol", "Age": 35}] -> Option B

4. Quick Check:

   Age > 30 returns Carol only [OK]

Hint: Filter returns only items strictly matching condition [OK]

Common Mistakes:

• Including items with Age equal to 30
• Confusing greater than with greater or equal
• Returning all items ignoring filter

4. You wrote this Scan code with filter expression: FilterExpression='Age > :val' and ExpressionAttributeValues={':val': 30}, but it returns no items. What is the likely error?

medium

A. ExpressionAttributeValues must be a dictionary with DynamoDB types, e.g., {':val': {'N': '30'}}.

B. FilterExpression should use '>' not '>'.

C. ExpressionAttributeValues key should be ':val' as a string, not a number.

D. Scan does not support FilterExpression.

Solution

1. Step 1: Check ExpressionAttributeValues format

   DynamoDB expects attribute values in a typed format, e.g., numbers as {'N': '30'}.

2. Step 2: Identify why no items returned

   Providing raw number 30 instead of typed value causes filter to fail matching any item.

3. Final Answer:

   ExpressionAttributeValues must be a dictionary with DynamoDB types, e.g., {':val': {'N': '30'}}. -> Option A

4. Quick Check:

   Use typed values in ExpressionAttributeValues [OK]

Hint: Use DynamoDB typed values in ExpressionAttributeValues [OK]

Common Mistakes:

• Passing raw Python values instead of typed dict
• Using HTML entities like > in code instead of >
• Thinking Scan does not support filters

5. You want to scan a DynamoDB table to find items where Status is 'Active' and Score is greater than 80. Which filter expression and attribute values are correct?

hard

A. FilterExpression='Status = :s AND Score > :sc', ExpressionAttributeValues={':s': {'S': 'Active'}, ':sc': {'N': '80'}}

B. FilterExpression='Status == :s AND Score > :sc', ExpressionAttributeValues={':s': 'Active', ':sc': 80}

C. FilterExpression='Status = :s OR Score > :sc', ExpressionAttributeValues={':s': {'S': 'Active'}, ':sc': {'N': '80'}}

D. FilterExpression='Status = :s AND Score > :sc', ExpressionAttributeValues={':s': 'Active', ':sc': '80'}

Solution

1. Step 1: Check filter expression logic

   The condition requires both Status equals 'Active' AND Score greater than 80, so use 'AND' and '=' operators correctly.

2. Step 2: Verify ExpressionAttributeValues format

   Values must be typed: strings as {'S': 'Active'} and numbers as {'N': '80'}.

3. Final Answer:

   FilterExpression='Status = :s AND Score > :sc', ExpressionAttributeValues={':s': {'S': 'Active'}, ':sc': {'N': '80'}} -> Option A

4. Quick Check:

   Correct logic and typed values [OK]

Hint: Use AND and typed values for multiple conditions [OK]

Common Mistakes:

• Using '==' instead of '=' in filter expression
• Using OR instead of AND
• Passing untyped values in ExpressionAttributeValues