The first rule is simply a repeat of the earlier structure requirement that each row in a table be unique. The entity integrity rule requires that no part of the primary key of a table be allowed to accept null values. Blank data fields do not lend themselves to an adequate description of the row, so requiring data elements in the column or columns that make up the key to the row makes perfect sense. Requiring that the key for each row be unique fulfills the mission of the relational model.
Chapter I -Introduction to the Relational Datobase
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