Question

what is primary key ?​

Answer 1

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Each table in a relational database must be assigned a primary key. The primary key is a column, or set of columns, whose values uniquely identify each row in the table.

Answer 2

In the relational model of databases, a primary key is a specific choice of a minimal set of attributes (columns) that uniquely specify a tuple (row) in a relation (table). Informally, a primary key is "which attributes identify a record", and in simple cases are simply a single attribute: a unique id.

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