How to Use Update Expression in DynamoDB: Syntax and Example

In DynamoDB, use the UpdateExpression parameter to specify attribute changes like setting, adding, or removing values. Combine it with ExpressionAttributeNames and ExpressionAttributeValues to safely reference attributes and values in your update operation.

Syntax

The UpdateExpression defines how to modify attributes in an item. It supports keywords like SET, REMOVE, ADD, and DELETE. Use ExpressionAttributeNames to substitute attribute names and ExpressionAttributeValues to provide values safely.

Example parts:

• SET: Assign or update attribute values.
• REMOVE: Delete attributes.
• ADD: Add numbers or append to sets.
• DELETE: Remove elements from sets.

plaintext

UpdateExpression = "SET #attrName = :val"
ExpressionAttributeNames = {"#attrName": "AttributeName"}
ExpressionAttributeValues = {":val": {"S": "NewValue"}}

Example

This example updates a user's age and adds a new tag to their tags set in a DynamoDB table named Users. It shows how to use UpdateExpression with placeholders for attribute names and values.

python

import boto3

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

# Update item in Users table
response = client.update_item(
    TableName='Users',
    Key={
        'UserId': {'S': 'user123'}
    },
    UpdateExpression='SET #age = :newAge ADD #tags :newTag',
    ExpressionAttributeNames={
        '#age': 'Age',
        '#tags': 'Tags'
    },
    ExpressionAttributeValues={
        ':newAge': {'N': '30'},
        ':newTag': {'SS': ['premium']}
    },
    ReturnValues='UPDATED_NEW'
)

print(response['Attributes'])

Output

{"Age": {"N": "30"}, "Tags": {"SS": ["premium"]}}

Common Pitfalls

Common mistakes when using UpdateExpression include:

• Not using ExpressionAttributeNames for reserved words or special characters in attribute names.
• Forgetting to prefix attribute names with # and values with : in the expression.
• Using incorrect data types in ExpressionAttributeValues.
• Not specifying ReturnValues if you want to see updated attributes.

Always test your update expressions carefully to avoid syntax errors or unexpected updates.

plaintext

Wrong:
UpdateExpression='SET Age = :newAge'
ExpressionAttributeValues={':newAge': {'N': '30'}}

Right:
UpdateExpression='SET #age = :newAge'
ExpressionAttributeNames={'#age': 'Age'}
ExpressionAttributeValues={':newAge': {'N': '30'}}

Quick Reference

Keyword	Purpose	Example
SET	Assign or update attribute values	SET #name = :val
REMOVE	Delete attributes from item	REMOVE #oldAttr
ADD	Add numbers or append to sets	ADD #count :inc
DELETE	Remove elements from sets	DELETE #tags :oldTags

Key Takeaways

Use UpdateExpression with SET, REMOVE, ADD, or DELETE to modify DynamoDB item attributes.

Always use ExpressionAttributeNames and ExpressionAttributeValues to avoid conflicts and injection.

Prefix attribute names with # and values with : in your update expressions.

Specify ReturnValues='UPDATED_NEW' to get the updated attributes after the update.

Test update expressions carefully to avoid syntax errors and data type mismatches.