Date_start is not a primary key in and of itself although it is one quarter of the primary key of the 'entity_values' table along with the the entities.entity_id, measures.measure_id, and and entity_values.date_end.
This field is used to indicate the start of the date range which the values in the 'entity_values' table represent.
It is also used to link the values to their footnotes via the 'footnote_values' table.
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