UpdateItem basics in DynamoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the result of this UpdateItem operation?

Given a DynamoDB table Users with a primary key UserID and an attribute Age, what will be the new value of Age after running this UpdateItem?

{
  TableName: "Users",
  Key: { "UserID": { "S": "user123" } },
  UpdateExpression: "SET Age = Age + :inc",
  ExpressionAttributeValues: { ":inc": { "N": "5" } },
  ReturnValues: "UPDATED_NEW"
}

Assume the current Age is 30.

DynamoDB

TableName: "Users"
Key: { "UserID": { "S": "user123" } }
UpdateExpression: "SET Age = Age + :inc"
ExpressionAttributeValues: { ":inc": { "N": "5" } }
ReturnValues: "UPDATED_NEW"

A {"Age": 35}

B {"Age": 5}

C {"Age": 30}

D SyntaxError

💡 Hint

Think about how the SET expression updates the existing value.

The UpdateExpression adds 5 to the current Age of 30, resulting in 35.

📝 Syntax

intermediate

Which UpdateExpression syntax is valid to set a new attribute?

Which of the following UpdateExpression syntaxes correctly adds a new attribute Status with value Active?

A SET Status = :status

B ADD Status = 'Active'

C SET Status = Active

D UPDATE Status = 'Active'

💡 Hint

Remember to use ExpressionAttributeValues for values in UpdateExpression.

Option A uses a placeholder :status which should be defined in ExpressionAttributeValues. This is the correct syntax.

❓ query_result

advanced

What happens if you try to increment a non-existing numeric attribute?

Consider this UpdateItem request on a DynamoDB table where the attribute Score does not exist yet for the item:

{
  TableName: "Games",
  Key: { "GameID": { "S": "game789" } },
  UpdateExpression: "SET Score = Score + :val",
  ExpressionAttributeValues: { ":val": { "N": "10" } },
  ReturnValues: "UPDATED_NEW"
}

What will be the result?

A Score attribute is created with value 10

B Update fails with a ConditionalCheckFailedException

C Score attribute remains undefined

D Update fails with a ValidationException

💡 Hint

Think about how DynamoDB handles arithmetic on missing attributes.

DynamoDB throws a ValidationException if you try to add a number to a non-existing attribute without initializing it first.

🔧 Debug

advanced

Why does this UpdateItem request fail with a syntax error?

Examine this UpdateItem snippet:

{
  TableName: "Employees",
  Key: { "EmpID": { "S": "e123" } },
  UpdateExpression: "SET #n = :name",
  ExpressionAttributeNames: { "#n": "Name" },
  ExpressionAttributeValues: { ":name": "John" },
  ReturnValues: "UPDATED_NEW"
}

Why does it fail?

A ExpressionAttributeNames key #n is invalid

B ExpressionAttributeValues value for :name is missing type wrapper

C UpdateExpression syntax is incorrect

D ReturnValues value is invalid

💡 Hint

Check the format of ExpressionAttributeValues.

ExpressionAttributeValues must specify the data type, e.g., { ":name": { "S": "John" } }.

🧠 Conceptual

expert

Which UpdateItem option ensures atomic increment of a numeric attribute?

You want to safely increment a numeric attribute Count in a DynamoDB item without overwriting concurrent updates. Which UpdateItem approach guarantees atomic increment?

A Read the item, increment Count in code, then write back with PutItem

B Use UpdateExpression: "SET Count = Count + :inc" with ExpressionAttributeValues {":inc": {"N": "1"}}

C Use UpdateExpression: "ADD Count :inc" with ExpressionAttributeValues {":inc": {"N": "1"}}

D Use ConditionExpression to check Count before updating with SET

💡 Hint

Consider DynamoDB's atomic update operators.

The ADD operator atomically increments numeric attributes even if they don't exist yet.