Expressions with SDK helpers 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 attribute counter = 5, what will be the value of counter after running this update expression using AWS SDK helpers?

UpdateExpression: 'SET counter = counter + :inc'
ExpressionAttributeValues: { ':inc': 3 }

A counter = 8

B counter = 3

C counter = 5

D SyntaxError: invalid update expression

💡 Hint

Think about how the SET operation with addition works in DynamoDB update expressions.

The update expression adds the value of ':inc' (3) to the existing 'counter' value (5), resulting in 8.

📝 Syntax

intermediate

Which option is a valid DynamoDB condition expression using SDK helpers?

You want to check if the attribute status equals active before updating an item. Which condition expression is valid?

A ConditionExpression: 'status == :val', ExpressionAttributeValues: { ':val': 'active' }

B ConditionExpression: 'status = :val', ExpressionAttributeValues: { ':val': 'active' }

C ConditionExpression: '#st = :val', ExpressionAttributeNames: { '#st': 'status' }, ExpressionAttributeValues: { ':val': 'active' }

D ConditionExpression: 'status equals :val', ExpressionAttributeValues: { ':val': 'active' }

💡 Hint

Remember that attribute names sometimes need placeholders and the equality operator is a single equals sign.

Option C correctly uses an attribute name placeholder '#st' and the '=' operator for equality. Option C misses the placeholder which is required if 'status' is a reserved word or to avoid ambiguity. Options C and D use invalid operators.

❓ optimization

advanced

Which update expression is the most efficient to append an element to a list attribute tags?

You want to add the string 'new' to the end of the tags list attribute in a DynamoDB item. Which update expression is best?

A UpdateExpression: 'ADD tags :newTag', ExpressionAttributeValues: { ':newTag': ['new'] }

B UpdateExpression: 'SET tags = list_append(tags, :newTag)', ExpressionAttributeValues: { ':newTag': ['new'] }

C UpdateExpression: 'SET tags = append(tags, :newTag)', ExpressionAttributeValues: { ':newTag': ['new'] }

D UpdateExpression: 'SET tags = tags + :newTag', ExpressionAttributeValues: { ':newTag': ['new'] }

💡 Hint

Check which function is supported by DynamoDB to append to lists.

DynamoDB supports the list_append function to concatenate lists. Option B uses it correctly. Option B uses '+' which is invalid for lists. Option B uses ADD which is for sets or numbers, not lists. Option B uses a non-existent function 'append'.

🔧 Debug

advanced

What error will this DynamoDB update expression cause?

Consider this update expression:
UpdateExpression: 'SET #name = :val'
ExpressionAttributeValues: { ':val': 'John' }
But ExpressionAttributeNames is missing. What happens?

A SyntaxError: invalid expression syntax

B TypeError: ExpressionAttributeValues missing key ':name'

C No error, updates attribute named '#name' literally

D ValidationException: ExpressionAttributeNames missing for placeholder '#name'

💡 Hint

Placeholders starting with '#' must be defined in ExpressionAttributeNames.

Using '#name' without defining it in ExpressionAttributeNames causes a ValidationException because DynamoDB cannot resolve the placeholder.

🧠 Conceptual

expert

Which statement about DynamoDB condition expressions with SDK helpers is TRUE?

Select the correct statement about using condition expressions with AWS SDK helpers in DynamoDB.

A Condition expressions support logical operators like AND, OR, and NOT to combine multiple conditions.

B You must always use ExpressionAttributeNames for all attribute names in condition expressions.

C Condition expressions cannot check for attribute existence or absence.

D Condition expressions can only compare attributes to constant values, not to other attributes.

💡 Hint

Think about how you combine multiple conditions in DynamoDB.

DynamoDB condition expressions support logical operators AND, OR, and NOT to combine multiple conditions. Option A is false because you can compare attributes to other attributes. Option A is false because ExpressionAttributeNames are only needed when attribute names are reserved words or ambiguous. Option A is false because functions like attribute_exists and attribute_not_exists are supported.