Question

I have the following table, which enforces a foreign key on vendor_id:

CREATE TABLE `notes` (
  `vendor_id` varchar(20) NOT NULL DEFAULT '',
  KEY `vendor_id` (`vendor_id`),
  CONSTRAINT `notes_ibfk_1` FOREIGN KEY (`vendor_id`) REFERENCES `title` (`vendor_id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

I was looking over someone else’s table, and it had the KEY, but did not show a CONSTRAINT, as follows:

CREATE TABLE `notes` (
  `vendor_id` varchar(20) NOT NULL DEFAULT '',
  KEY `vendor_id` (`vendor_id`),
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

What exactly does this mean / do to have a KEY without anything else?

Answer 1

KEY is a synonym for INDEX, so in this case, vendor_id has had an index created on it, but has not specifically been defined as the table’s PRIMARY KEY, which would enforce uniqueness.

From the MySQL CREATE TABLE docs:

KEY is normally a synonym for INDEX. The key attribute PRIMARY KEY can also be specified as just KEY when given in a column definition. This was implemented for compatibility with other database systems.