Keys

A minimal set of attributes that uniquely identifies the tuple

(I.e., there is no pair of tuples with the same values for the

key attributes):

Person: social security number

name

name + address

name + address + age

Perfect keys are often hard to find, but organizations usually

invent something anyway.

Superkey: a set of attributes that contains a key.

A relation may have multiple keys: (but only one primary key)

employee number, social-security number

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