Generate unique and random code digit with 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;

Answer 2

(I understand that this way of generating is a kind of proof of concept)

The very strange thing is how this request could return an already existing value????