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Core
Slowly Changing Dimensions (SCD)
Slowly changing dimensions are the patterns for handling dimension attributes that change over time, such as a customer moving cities or a product changing category. Type 1 overwrites history, Type 2 retains versioned rows with effective dates and a current flag, and Type 3 retains a prior-value column. AI, ML, and GenAI interviews probe it because answering what something looked like at the time of an event requires deliberate history tracking, and most analysts only know how to overwrite.
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