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.