- Every row in the changes report has an Effective Date. The values in every row correspond to the effective date.
- For fields that come from a history table (like title, department, salary etc.) we know when each value started to be effective and when it stopped.
- For fields that don't have an effective date (like display name, gender etc.) we assume a value is effective from the moment it's entered and until it's removed or changed.
If an employee is created as "married" for example, we assume the employee was married at the time it was added to Bob (the employee creation date). We don't assume anything about the marital status of the employee prior to that date, because we don't know if the employee got married one day or 30 years before being added to Bob. So we leave it empty in rows corresponding to a date before the employee's creation date.
The same logic applies to display name (and often those two are changed together) and all other fields without an effective date.