Expression attribute values in DynamoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this DynamoDB update expression?

Given a DynamoDB table with an item having id = 101 and attribute score = 50, what will be the new value of score after running this update?

UpdateExpression: "SET score = score + :inc"
ExpressionAttributeValues: {":inc": {"N": "20"}}

A The score will be updated to 70

B The score will be updated to 20

C The update will fail due to missing attribute

D The score will remain 50

💡 Hint

Remember that :inc is a placeholder for the value 20 used in the update expression.

The update expression adds the value of :inc (which is 20) to the existing score (which is 50), resulting in 70.

📝 Syntax

intermediate

Which ExpressionAttributeValues syntax is correct for a string value?

You want to update an attribute status to the string value active. Which of the following ExpressionAttributeValues is valid?

A { ":val": { "BOOL": "active" } }

B { ":val": "active" }

C { ":val": { "N": "active" } }

D { ":val": { "S": "active" } }

💡 Hint

DynamoDB expects data types like S for string, N for number, BOOL for boolean.

The correct syntax for a string value uses the key "S" with the string as its value.

❓ optimization

advanced

How to efficiently update multiple attributes using ExpressionAttributeValues?

You want to update attributes age to 30 and city to "Seattle" in one update call. Which ExpressionAttributeValues and UpdateExpression combination is correct and efficient?

A UpdateExpression: "SET age = :a, city = :c"<br>ExpressionAttributeValues: {":a": {"N": "30"}, ":c": {"S": "Seattle"}}

B UpdateExpression: "SET age = 30, city = 'Seattle'"<br>ExpressionAttributeValues: {}

C UpdateExpression: "SET age = :age, city = :city"<br>ExpressionAttributeValues: {":age": 30, ":city": "Seattle"}

D UpdateExpression: "SET age = :a, city = :c"<br>ExpressionAttributeValues: {":a": 30, ":c": "Seattle"}

💡 Hint

ExpressionAttributeValues must specify data types for values.

Option A correctly uses placeholders with proper DynamoDB data types and a valid update expression.

🔧 Debug

advanced

Why does this update fail with a ValidationException?

Given this update:

UpdateExpression: "SET name = :n"
ExpressionAttributeValues: {":n": {"N": "John"}}

Why does DynamoDB return a ValidationException error?

A Because the placeholder ":n" is missing in the UpdateExpression

B Because the data type "N" is used for a string value instead of "S"

C Because the attribute name "name" is a reserved word and needs aliasing

D Because ExpressionAttributeValues cannot contain nested objects

💡 Hint

Check the data type used for the value "John".

The value "John" is a string but the data type "N" (number) was used, causing a type mismatch error.

🧠 Conceptual

expert

What is the purpose of ExpressionAttributeValues in DynamoDB operations?

Choose the best explanation for why ExpressionAttributeValues are used in DynamoDB queries and updates.

A They store the results returned by DynamoDB after a query or scan operation

B They define the schema of the table and enforce data types on attributes

C They act as placeholders to safely substitute values in expressions, preventing injection and syntax errors

D They are used to create indexes on attributes for faster querying

💡 Hint

Think about how placeholders help in writing safe and reusable expressions.

ExpressionAttributeValues provide a way to safely insert values into expressions without risking injection or syntax errors.