Creating a partial unique constraint for MySQL

I have the same question as asked in a previous post: PostgreSQL multi-column unique constraint and NULL values. But the solution there is not applicable as I am using MySQL instead of PostgreSQL.

My table looks like this:

CREATE TABLE `my_table` (
  `id` bigint(20) NOT NULL,
  `col1` bigint(20) NOT NULL,
  `col2` bigint(20) NOT NULL,
  `col3` bigint(20) DEFAULT NULL,
  PRIMARY KEY (`id`)
)

I want the three columns col1, col2 and col3 to be unique. But a normal unique constraint

UNIQUE KEY `unique_constraint` (`col1`,`col2`,`col3`)

still allows me to insert identical rows whose col3 is NULL. I want to forbid entries like these:

1  | 1 | 1 | NULL
2  | 1 | 1 | NULL

I was searching for a partial unique constraint in MySQL as proposed in the post above but apparently there are not supported in MySQL 5.5.

Can somebody give me a hint how to solve this best in mysql, I am also hoping there will be a performant solution!

Thanks from a newbie in mysql (coming from PostgreSQL)!

The only option that comes to mind is a BEFORE INSERT trigger. In 5.5 you could use SIGNAL:

DELIMITER $
CREATE TRIGGER bi_foo BEFORE INSERT on foo
FOR EACH ROW
BEGIN
  IF (ISNULL(NEW.col3) AND (SELECT COUNT(*) FROM foo 
     WHERE col1 = NEW.col1 AND col2 = NEW.col2 AND col3 IS NULL) > 0)
  THEN
    SIGNAL SQLSTATE '45000'
     SET MESSAGE_TEXT = 'Cannot insert this item due to unique constraint', 
         MYSQL_ERRNO = 1001;
  END IF;
END$
DELIMITER ;

[sqlfiddle]

But pre 5.5, you would do this something like this:

DELIMITER $
CREATE TRIGGER bi_foo BEFORE INSERT on foo
FOR EACH ROW
BEGIN
  IF (ISNULL(NEW.col3) AND (SELECT COUNT(*) FROM foo 
     WHERE col1 = NEW.col1 AND col2 = NEW.col2 AND col3 IS NULL) > 0)
  THEN
    SET NEW='Error: Cannot insert this item due to unique constraint';
  END IF;
END$
DELIMITER ;

[sqlfiddle]

StackExchange Database Administrators Q#41030, answer score: 7