Why we cannot create more than one primary key on a table. Why we create several unique key on a table. Please explain if anyone have details of this.

Why we cannot create more than one primary key on a table. Why we create several unique key on a table. Please explain if anyone have details of this.


«a primary key has semantic meaning, it is to be immutable (never change of value), unique and not null.»

a unique constraint is simply "at any time, these values are unique - they can change and they can be null.

You use a unique when constraint

(a) you do not already have a primary key for a table can have only one
(b) you allow NULL values in attributes
"(c) to allow you to update the values in the attributes.


Tags: Database

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