Which key can not be null?


The primary key serves as a unique identifier for rows in the table. Entity IntegrityEntity IntegrityEntity integrity is concerned with ensuring that each row of a table has a unique and non-null primary key value; this is the same as saying that each row in a table represents a single instance of the entity type modelled by the table.https://en.wikipedia.org › wiki › Entity_integrityEntity integrity – Wikipedia ensures two properties for primary keys: The primary key for a row is unique; it does not match the primary key of any other row in the table. The primary key is not null, no component of the primary key may be set to null.

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