How to Use SET in Update Expression in DynamoDB

In DynamoDB, use the SET clause in an update expression to assign new values to one or more attributes of an item. The syntax is SET attributeName = :value, where :value is a placeholder for the new value provided in ExpressionAttributeValues.

Syntax

The SET clause in a DynamoDB update expression assigns new values to attributes. It can update one or multiple attributes separated by commas.

• SET: Keyword to start the update assignment.
• attributeName: The attribute you want to update.
• = :value: Assigns the attribute to the value represented by a placeholder.
• ExpressionAttributeValues: A map of placeholders to actual values.

json

UpdateExpression = "SET attributeName = :value"
ExpressionAttributeValues = { ":value": {"S": "newValue"} }

Example

This example updates the status attribute of an item with primary key id to "completed" using the SET clause.

python

import boto3

# Initialize DynamoDB client
client = boto3.client('dynamodb')

# Update item with SET expression
response = client.update_item(
    TableName='Tasks',
    Key={
        'id': {'S': 'task123'}
    },
    UpdateExpression='SET #st = :newStatus',
    ExpressionAttributeNames={
        '#st': 'status'
    },
    ExpressionAttributeValues={
        ':newStatus': {'S': 'completed'}
    },
    ReturnValues='UPDATED_NEW'
)

print(response['Attributes'])

Output

{"status": {"S": "completed"}}

Common Pitfalls

Common mistakes when using SET in update expressions include:

• Not using ExpressionAttributeNames when attribute names are reserved words or contain special characters.
• Forgetting to define all placeholders used in UpdateExpression inside ExpressionAttributeValues.
• Using incorrect data types for values (e.g., string vs number).
• Not specifying ReturnValues if you want to see updated attributes.

text

Wrong:
UpdateExpression='SET status = :newStatus'
# Missing ExpressionAttributeValues for :newStatus

Right:
UpdateExpression='SET status = :newStatus'
ExpressionAttributeValues={':newStatus': {'S': 'completed'}}

Quick Reference

Part	Description	Example
SET	Keyword to assign new values	SET attributeName = :value
attributeName	Name of attribute to update	status
:value	Placeholder for new value	:newStatus
ExpressionAttributeValues	Map placeholders to actual values	{ ':newStatus': {'S': 'completed'} }
ExpressionAttributeNames	Map attribute name placeholders if needed	{ '#st': 'status' }

Key Takeaways

Use the SET clause in UpdateExpression to assign new values to attributes.

Always define placeholders in ExpressionAttributeValues for values used in SET.

Use ExpressionAttributeNames to avoid conflicts with reserved words.

Specify ReturnValues='UPDATED_NEW' to get updated attributes in response.

Separate multiple attribute updates with commas in the SET clause.