Difference between per_all_people_f "DATE_START" and per_periods_of_service "Date_start"

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Hi Everyone,

What is the Difference between per_all_people_f "DATE_START" and per_periods_of_service "Date_start" column.

From the table and view explanation i understand that per_periods_of_service .date_start is start date of employment and per_all_people_f .date_start is denormalised record. But i am not getting clear picture and understanding. Any explanation with real time scenarios would be highly helpful.

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