Change Detection Temporal Tables

Adam Machanic shows how to find net changes using temporal tables:

For now, consider the following set of propositions, given that we’re asking at time Y for all changes since a prior time X.

  • INSERT: The key did not exist at time X but does exist at time Y.
  • DELETE: The key existed at time X but does not exist at time Y.
  • UPDATE: The key existed at both time X and at time Y, and at least one change occurred between time X and time Y.

Given these assumptions, we can begin work on a temporal queries that return the necessary rows. Solving for these conditions will require all rows that were active as of time X, rows that were (or are) active at time Y, and for the final case, all rows that were active in-between times X and Y. Since this is a range-based scenario, our best Temporal predication option will be either FROM or BETWEEN. The difference between these two is subtle: FROM uses an open interval (non-inclusive at both endpoints), whereas BETWEEN uses a half-open interval, inclusive on the end date. Given the choice in the scenario, BETWEEN makes more sense, as we can take advantage of the inclusive endpoint to avoid dropping a badly-timed row. But more on that in a moment.

Adam put a lot of thought into edge cases, making this a must-read.

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