Generate unique and random code digit with MySQL - mysql

Question

Initial goal:
I would like to generate random and unique codes (6 digits) in a table.
I use a SQL query like this one to do that:
SELECT SUBSTRING(CRC32(RAND()), 1, 6) as myCode
FROM `codes`
HAVING myCode NOT IN (SELECT code FROM `codes`)
I asked me about how it will react when there will be no more available codes so I do the following test
Test context:
MySQL version: 5.5.20
MySQL Table:
CREATE TABLE `codes` (
`id` INT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY ,
`code` VARCHAR( 10 ) NOT NULL ,
UNIQUE (
`code`
)
) ENGINE = InnoDB;
Initial data:
INSERT INTO `codes` (`id`, `code`)
VALUES (NULL, '1'), (NULL, '2'), (NULL, '3'), (NULL, '4'), (NULL, '5'), (NULL, '6'), (NULL, '7'), (NULL, '8');
SQL Query:
SELECT SUBSTRING(CRC32(RAND()), 1, 1) as myCode
FROM `codes`
HAVING myCode NOT IN (SELECT code FROM `codes`)
By execute this query, I expect that it will always return 9 because it is the only code of one digit which does not exists.
But the result is:
Sometime it return any rows
Sometime it return rows with values that already exists
I don't understand this behavior so if someone can help :)
So the big question is:
How MySQL can return rows with values that already exists?
Thanks

Answer 1

I would fill a sequencetable table with all the possible values, in sequence.
Then the random query just randomly selects records from the sequencetable, and each time it picks a record it deletes it. This way you will surely get all the numbers, without wasting time in finding a "hole" number (not already picked up).
CREATE TABLE `sequencetable`
(
`sequence` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
PRIMARY KEY (`sequence`)
)
ENGINE=InnoDB
AUTO_INCREMENT=1;
Fill the sequence (no need for the AUTOINCREMENT actually).
DECLARE i INT;
SET i=1;
REPEAT
INSERT INTO sequencetable VALUES (i);
SET i=i+1;
UNTIL i>999999 END REPEAT;
Select a random record from the sequence (do this in a loop until records are available):
DECLARE sequencen INT;
SET sequencen =
(SELECT sequence FROM sequencetable ORDER BY RAND() LIMIT 1);
DELETE FROM sequencetable WHERE sequence = sequencen;