Using Condition Expressions

Syntax

Condition expressions are used to perform conditional writes in DynamoDB. The syntax for condition expressions is as follows:

{
    "ConditionExpression": "expression",
    "ExpressionAttributeValues": {
        "value1": "actualValue1",
        "value2": "actualValue2",
        ...
    }
}

Here, ConditionExpression is the expression that is evaluated before the write operation is performed. ExpressionAttributeValues is a map of variable names to actual values that are used in the condition expression.

Example

Suppose we want to update a customer's order only if the order has not already been fulfilled. We can use a condition expression to ensure that the order has not already been fulfilled before updating it.

{
    "TableName": "Orders",
    "Key": {
        "CustomerId": "123",
        "OrderId": "456"
    },
    "UpdateExpression": "SET DeliveryDate = :deliveryDate",
    "ConditionExpression": "attribute_not_exists(FulfillmentDate)",
    "ExpressionAttributeValues": {
        ":deliveryDate": "2022-01-01"
    }
}

Here, the ConditionExpression ensures that the FulfillmentDate attribute does not exist. If the FulfillmentDate attribute does exist, the update operation will not be performed.

Output

If the condition expression is true, the update operation will be performed and the response will contain details about the updated item. If the condition expression is false, the update operation will not be performed and the response will contain an error message.

Explanation

The condition expression is evaluated before the write operation is performed. If the condition expression evaluates to true, the write operation will be performed. If the condition expression evaluates to false, the write operation will not be performed.

Use

Condition expressions can be used to perform conditional writes in DynamoDB, ensuring that write operations only occur when certain conditions are met.

Important Points

• Condition expressions can be used with both put and update operations.
• It is important to ensure that the condition expression is correct and accurately reflects the desired condition, as otherwise updates may not occur as expected.
• Condition expressions must be written in DynamoDB expression syntax.

Summary

Condition expressions are an important feature of DynamoDB, allowing developers to perform conditional writes based on certain conditions being met. By using condition expressions, developers can ensure that data is only updated in certain situations, helping to maintain data integrity and accuracy.