public String getProjectionExpression() {
        return projectionExpression;
    }
A string that identifies one or more attributes of the specified item to retrieve from the table. The attributes
in the expression must be separated by commas. If no attribute names are specified, then all attributes of the
specified item are returned. If any of the requested attributes are not found, they do not appear in the result.
aws-java-sdk-dynamodb/src/main/java/com/amazonaws/services/dynamodbv2/datamodeling/DynamoDBTransactionLoadExpression.java