Can mySQL define specific columns as UNIQUE based on another column?

Question

Suppose I have a table:

  CREATE TABLE `ml_vendor_refs` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `ml_id` int(11) NOT NULL,
  `ven_id` int(11) NOT NULL,
  `designator` int(4) NOT NULL,
  `telco` tinyint(1) NOT NULL DEFAULT '0',
  PRIMARY KEY (`id`)
) ENGINE=MyISAM AUTO_INCREMENT=5 DEFAULT CHARSET=latin1

As you can see, "id" is primary and AI. However, I want to be able to add several rows of 'designator' that my not duplicate based on the parent of 'ml_id' and 'telco'. For example:

"id"    "ml_id" "ven_id"    "designator"    "telco"
"1"     "5144"    "3"            "1"           "0"
"2"     "5144"    "7"            "2"           "0"
"3"     "5144"    "44"           "3"           "0"
"4"     "5144"    "49"           "4"           "0"

for every instance of "ml_id" and the boolean "telco", I want to be able to have unique and ONLY unique "designators". Make sense?

Thanks

Answer 1

You can have a UNIQUE index than spans multiple columns. The result would be a restriction such that the same value can appear in one column multiple times but the same combination of values cannot appear in the specified columns.

CREATE TABLE `ml_vendor_refs` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `ml_id` int(11) NOT NULL,
  `ven_id` int(11) NOT NULL,
  `designator` int(4) NOT NULL,
  `telco` tinyint(1) NOT NULL DEFAULT '0',
  PRIMARY KEY (`id`),
  UNIQUE KEY `designators` (`designator`,`ml_id`,`telco`)
) ENGINE=MyISAM AUTO_INCREMENT=5 DEFAULT CHARSET=latin1

Answer 2

UNIQUE( ml_id, telco, designator )

Answer 3

You can have a unique pairing defined if you add a unique index on more than one column. For instance, you could add an index on ml_id, designator that forces the pairings to be unique.

As far as I know, NULL values are not subject to unique constraints.