Filter expressions in DynamoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Filter expression with attribute_exists

Given a DynamoDB table with items having attributes id and status, what will be the output of this filter expression?

FilterExpression: "attribute_exists(status) AND status = :val"

with :val set to "active"?

A Returns all items regardless of the <code>status</code> attribute.

B Returns only items where the attribute <code>status</code> exists and its value is exactly "active".

C Returns items where <code>status</code> does not exist or is "active".

D Returns items where <code>status</code> exists but its value is not "active".

💡 Hint

Think about what attribute_exists means and how it combines with the equality check.

The filter expression attribute_exists(status) AND status = :val means the item must have the status attribute and its value must be exactly "active". Items missing status or with a different value are excluded.

❓ query_result

intermediate

Using begins_with in filter expressions

What will be the result of this filter expression on a DynamoDB table with a username attribute?

FilterExpression: "begins_with(username, :prefix)"

with :prefix set to "user"?

A Returns items where the <code>username</code> ends with "user".

B Returns items where the <code>username</code> contains "user" anywhere.

C Returns items where the <code>username</code> starts with "user".

D Returns items where the <code>username</code> is exactly "user".

💡 Hint

Remember what the function begins_with checks in DynamoDB.

The begins_with function filters items whose attribute value starts exactly with the given prefix. It does not check for substring or suffix.

📝 Syntax

advanced

Identify the syntax error in this filter expression

Which option contains a syntax error in the DynamoDB filter expression?

Expression: FilterExpression: "status = :val AND (attribute_exists(age) OR age > :minAge"

Assume :val and :minAge are defined.

A Missing closing parenthesis in the filter expression causes a syntax error.

B Using <code>attribute_exists</code> inside parentheses is not allowed.

C Using comparison operators like <code>></code> in filter expressions is not allowed.

D Using <code>AND</code> and <code>OR</code> together is invalid syntax.

💡 Hint

Check if all parentheses are properly closed.

The filter expression has an opening parenthesis before attribute_exists(age) but no matching closing parenthesis. This causes a syntax error.

❓ optimization

advanced

Optimizing filter expressions for performance

Which filter expression is more efficient for filtering items where score is between 50 and 100 inclusive in DynamoDB?

A FilterExpression: "score BETWEEN :min AND :max" with <code>:min=50</code> and <code>:max=100</code>

B FilterExpression: "score >= :min AND score <= :max" with <code>:min=50</code> and <code>:max=100</code>

C FilterExpression: "score > :min AND score < :max" with <code>:min=49</code> and <code>:max=101</code>

D FilterExpression: "score IN (:min, :max)" with <code>:min=50</code> and <code>:max=100</code>

💡 Hint

Consider which expression is simpler and uses built-in operators.

The BETWEEN operator is optimized for range checks and is more readable and efficient than combining two comparisons with AND. The IN operator checks for exact matches, not ranges.

🧠 Conceptual

expert

Understanding filter expressions impact on read capacity

Which statement best describes how filter expressions affect DynamoDB read capacity units (RCUs) when scanning a table?

A Filter expressions cause DynamoDB to read only matching items, greatly reducing RCUs.

B Filter expressions reduce the number of items read from the table, lowering RCUs consumed.

C Filter expressions increase RCUs consumed because they add extra processing on the server.

D Filter expressions do not reduce the number of items read; they only filter results after reading, so RCUs consumed remain the same.

💡 Hint

Think about when filtering happens during a scan operation.

Filter expressions are applied after DynamoDB reads the items. They do not reduce the number of items read, so the RCUs consumed are based on the total items scanned, not filtered.

Practice

1. What is the main purpose of a FilterExpression in a DynamoDB scan or query?

easy

A. To return only items that meet specific conditions after reading all data

B. To reduce the size of the table permanently

C. To speed up the write operations

D. To create new indexes automatically

Solution

1. Step 1: Understand what FilterExpression does

   A FilterExpression is used to specify conditions that items must meet to be returned after the scan or query reads the data.

2. Step 2: Differentiate from other operations

   It does not change the table or speed up writes; it only filters results after reading.

3. Final Answer:

   To return only items that meet specific conditions after reading all data -> Option A

4. Quick Check:

   FilterExpression filters results after reading [OK]

Hint: FilterExpression filters results after reading data [OK]

Common Mistakes:

• Thinking FilterExpression reduces read capacity usage
• Confusing FilterExpression with key condition
• Assuming it changes the table data

2. Which of the following is the correct syntax for a FilterExpression to find items where attribute status equals 'active' in DynamoDB?

easy

A. FilterExpression: "status = 'active'"

B. FilterExpression: "status == 'active'"

C. FilterExpression: "status equals 'active'"

D. FilterExpression: "status === 'active'"

Solution

1. Step 1: Recall DynamoDB FilterExpression syntax

   DynamoDB uses single equals sign (=) for equality in FilterExpressions.

2. Step 2: Check each option

   FilterExpression: "status = 'active'" uses correct syntax. The other options use invalid operators (==, equals, ===) or keywords.

3. Final Answer:

   FilterExpression: "status = 'active'" -> Option A

4. Quick Check:

   Use single equals (=) for equality in FilterExpression [OK]

Hint: Use single '=' for equality in FilterExpression [OK]

Common Mistakes:

• Using double or triple equals (==, ===)
• Writing 'equals' as a word
• Using incorrect operators

3. Given a DynamoDB table with items having attribute age, what will be the result of this scan with FilterExpression age < 30 if the table has ages 25, 30, 35, and 40?

medium

A. Items with ages 25 and 30

B. Items with ages 25 only

C. Items with ages 30, 35, and 40

D. Items with ages 35 and 40

Solution

1. Step 1: Understand the FilterExpression condition

   The condition is age < 30, meaning only items with age less than 30 pass the filter.

2. Step 2: Check which ages satisfy the condition

   From the list (25, 30, 35, 40), only 25 is less than 30.

3. Final Answer:

   Items with ages 25 only -> Option B

4. Quick Check:

   age < 30 means less than 30 [OK]

Hint: Remember < means strictly less than [OK]

Common Mistakes:

• Including age 30 when condition is less than 30
• Confusing < with <= operator
• Assuming filter changes data in table

4. You wrote this FilterExpression: contains(name, 'John') but it returns no results even though some items have 'John' in their name. What is the likely mistake?

medium

A. Using contains() instead of equals()

B. Not using ExpressionAttributeNames for reserved words

C. FilterExpression is case-sensitive and 'John' does not match 'john'

D. FilterExpression must be in uppercase

Solution

1. Step 1: Understand contains() behavior

   contains() checks if the attribute contains the exact substring, including case.

2. Step 2: Consider case sensitivity

   If the data has 'john' lowercase but the filter uses 'John' with uppercase J, no match occurs.

3. Step 3: Check other choices

   Using equals() instead of contains() is not the issue since contains() is appropriate for substrings; reserved words would cause a different error; FilterExpressions are not required to be uppercase.

4. Final Answer:

   FilterExpression is case-sensitive and 'John' does not match 'john' -> Option C

5. Quick Check:

   contains() is case-sensitive [OK]

Hint: Remember contains() is case-sensitive [OK]

Common Mistakes:

• Ignoring case sensitivity in contains()
• Not using ExpressionAttributeNames when needed
• Assuming contains() works like equals()

5. You want to scan a DynamoDB table to find items where score is greater than 50 and status is 'active'. Which FilterExpression correctly applies both conditions?

hard

A. FilterExpression: "score > 50, status = 'active'"

B. FilterExpression: "score > 50 OR status = 'active'"

C. FilterExpression: "score > 50 && status == 'active'"

D. FilterExpression: "score > 50 AND status = 'active'"

Solution

1. Step 1: Understand logical operators in FilterExpression

   DynamoDB uses uppercase AND and OR for combining conditions.

2. Step 2: Check each option's syntax

   FilterExpression: "score > 50 AND status = 'active'" uses correct syntax with AND and single equals. FilterExpression: "score > 50 OR status = 'active'" uses OR which is incorrect for both conditions. FilterExpression: "score > 50, status = 'active'" uses comma which is invalid. FilterExpression: "score > 50 && status == 'active'" uses && and double equals which are invalid.

3. Final Answer:

   FilterExpression: "score > 50 AND status = 'active'" -> Option D

4. Quick Check:

   Use AND in uppercase and single '=' for equality [OK]

Hint: Use uppercase AND and single '=' in FilterExpression [OK]

Common Mistakes:

• Using OR instead of AND for both conditions
• Using commas or && instead of AND
• Using double equals (==) instead of single equals (=)