I'm working on a project where every month, each of the 100 customers are asked various questions in a survey. The customers are very reliable in responding each month, but occasionally a few will miss a month or only answer a few of the questions. We also lose and gain new customers each month too.
We will be doing a full reload each month of the whole data set, but if a customer is missing the survey for the most recent month, then we need to use the values from the previous month they had completed.
What would be the best way to implement this reload method where we can do a full reload, yet still use old values if current values are missing? I should stress that only a few of the fields in each record may be missing, not the whole record. I'd need to find which fields are missing values, and replace them with older values.