Sql to Generate Random unique number

best

SELECT uuid() AS 'rvkey' FROM `{$prefix}visit` WHERE 'rvkey' NOT IN (SELECT `vkey` FROM `{$prefix}visit`) LIMIT 1

random_charachter

SELECT SUBSTRING(MD5(RAND()) FROM 1 FOR 8) AS random_charachter
FROM `table`
WHERE "random_charachter" NOT IN (SELECT `field` FROM `table`)
LIMIT 1

random_num

SELECT FLOOR(RAND() * 9999999) AS random_num
FROM `table`
WHERE "random_num" NOT IN (SELECT `field` FROM `table`)
LIMIT 1