SET expression for adding/changing in DynamoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the result of this SET expression update?

Given a DynamoDB item with attributes {"id": "123", "count": 5, "tags": ["blue"]}, what will be the updated item after applying this UpdateExpression?

SET count = count + :inc, tags = list_append(tags, :newTags)

with ExpressionAttributeValues:
{":inc": 3, ":newTags": ["green"]}

A {"id": "123", "count": 8, "tags": ["green", "blue"]}

B {"id": "123", "count": 3, "tags": ["green"]}

C {"id": "123", "count": 8, "tags": ["blue", "green"]}

D {"id": "123", "count": 5, "tags": ["blue"]}

💡 Hint

Remember that SET can add numbers and list_append appends to the end of a list.

The count attribute is increased by 3 (5 + 3 = 8). The tags list appends 'green' to the end, so it becomes ["blue", "green"].

📝 Syntax

intermediate

Which SET expression is syntactically correct for updating multiple attributes?

Choose the correct DynamoDB UpdateExpression to set attribute 'status' to 'active' and increment 'score' by 10.

A SET status = :s, score = score + :inc

B SET status := :s, score += :inc

C SET status = :s; score = score + :inc

D SET status == :s, score = score + :inc

💡 Hint

Use single equals '=' for assignment and '+' for addition in SET expressions.

Option A uses correct syntax: '=' for assignment and '+' for addition. Other options use invalid operators or punctuation.

❓ optimization

advanced

Optimizing a SET expression to update nested attributes

You want to update the nested attribute 'address.city' to 'Seattle' and increment 'visits' by 1 in a DynamoDB item. Which UpdateExpression is the most efficient and correct?

A SET address.city = :city, visits = visits + :one

B SET #addr.#city = :city, visits = visits + :one

C SET address['city'] = :city, visits = visits + :one

D SET address.city := :city, visits += :one

💡 Hint

Use ExpressionAttributeNames for nested attributes to avoid reserved word conflicts.

Option B uses ExpressionAttributeNames (#addr and #city) which is best practice for nested attributes. Option B may fail if 'address' or 'city' are reserved words. Options C and D have invalid syntax.

🔧 Debug

advanced

Why does this SET expression cause an error?

Given this UpdateExpression:
SET count = count + :inc, tags = list_append(:newTags, tags)
and ExpressionAttributeValues:
{":inc": 2, ":newTags": ["red"]}
The update fails with a ValidationException. Why?

A list_append requires the existing list as the first argument, but :newTags is first here.

B The syntax of list_append is incorrect; it needs three arguments.

C The ExpressionAttributeValues are missing a value for :tags.

D The attribute 'count' does not exist, so it cannot be incremented.

💡 Hint

Check the order of arguments in list_append function.

list_append(list1, list2) requires the existing list attribute as the first argument. Here, :newTags is first, causing the error.

🧠 Conceptual

expert

What happens when you use SET to add a new attribute that already exists?

If you run this UpdateExpression:
SET newAttr = if_not_exists(newAttr, :start) + :inc
with ExpressionAttributeValues:
{":start": 0, ":inc": 5}
and the item already has newAttr = 10, what will be the new value of newAttr?

A 5

B 0

C An error occurs because newAttr already exists.

D 15

💡 Hint

if_not_exists returns the attribute value if it exists, otherwise the default.

Since newAttr exists with value 10, if_not_exists returns 10. Then 10 + 5 = 15 is assigned.