Re: Question related to unique key

[Rick James]

Think of a PRIMARY KEY as the preferred way to unambiguously identify each row in the table. Ok, maybe you could envision a row as being identified by 'null', but why bother.

Another note: KEY is the same as INDEX. Neither implies uniqueness.

A common PRIMARY KEY is
INT UNSIGNED AUTO_INCREMENT NOT NULL
This assigns a number (1,2,...) to each row (assuming you don't explicitly set it).

An INSERT will spit at you if you violate a UNIQUEness constraint.

MyISAM does not require any keys.
InnoDB requires a PRIMARY KEY, even to the point of providing one if you don't specify one.