Slowly Changing Data and History Tracking in SQL
Model and query changing records over time.
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Help me choose how to track changing records over time. Database: [DATABASE]. Entity and attributes that change: [ENTITY_AND_CHANGING_ATTRIBUTES]. Questions I need to answer about the past: [HISTORY_QUESTIONS]. Update frequency and volume: [UPDATE_VOLUME]. Use this tree for each attribute: 1. Do reports need the value as it was at the time of the event? If no, overwrite the value (type 1), and note that history is lost. 2. If yes, do I need the full history of changes? Use effective-dated rows (type 2): add valid_from, valid_to, and is_current columns, with a new surrogate key per version. 3. Do I only need the current and the previous value? Consider separate columns (type 3). 4. Do changes happen very often or in a small set of attributes? Consider a separate history table or a mini-dimension. 5. Does the database offer temporal tables or change data capture? Explain the benefit. Deliver for my case: the table design, SQL to detect changes and expire old rows then insert new versions (a merge statement for my database), queries to get "as of" a date, the current view, and the join pattern from facts to the right dimension version using the event date. Include tests for overlapping date ranges, gaps, and duplicate current rows, and a note on late-arriving data.
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