Because in Data warehousing historicaldata should be maintained, to maintain historical data means suppose oneemployee details like where previously he worked, and now where he is working, all details should be maintain in one table, if u maintain primary key it won't allow the duplicaterecords with same employee id. so to maintain historical data we are all going for concept data warehousing by using surrogate keys we can achieve the historical data(using oracle sequence for critical column).
so all the dimensions are marinating historical data, they are de normalized, because of duplicate entry means not exactly duplicate record with same employee number another record is maintaining in the table.
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