SQL Unique Key Syntax

问题

Very basic question; I'm very new to SQL and trying to decipher an example data base.

In the below create table code, why does the define primary key syntax reference only the 'id' column once in parentheses but the unique key definition references the 'category' column twice? both before and within the parentheses.

Seems like there is a simple answer to this but cant track one down:

CREATE TABLE `categories` (
  `id` SMALLINT NOT NULL AUTO_INCREMENT,
  `category` VARCHAR(30) NOT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `category` (`category`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8;

回答1:

It is the key name, and it's not mandatory. See the doc about it : http://dev.mysql.com/doc/refman/5.1/en/create-table.html.

来源：https://stackoverflow.com/questions/17197473/sql-unique-key-syntax