Question

define primary key ?
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Answer 1

Answer:

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

Answer 2

Answer:

A Primary key is a column in a table that uniquely identifies each row in that particular table.