CREATE FUNCTION generate_password (p_alphabet VARCHAR(255), p_length INT)
RETURNS VARCHAR(255)
RETURN (WITH RECURSIVE
        cte AS ( SELECT 1 n UNION ALL SELECT n + 1 FROM cte WHERE n < p_length)
        SELECT GROUP_CONCAT(SUBSTRING(p_alphabet FROM CEIL(RAND() * LENGTH(p_alphabet)) FOR 1) SEPARATOR '')
        FROM cte);

CREATE FUNCTION generate_and_check()
RETURNS CHAR(8)
BEGIN
    DECLARE unique_key CHAR(8);
    REPEAT
        SET unique_key := generate_password('qwertyuiopasdfghjklzxcvbnm0123456879', 8);
    UNTIL !COALESCE(( SELECT COUNT(*) FROM mytable WHERE key_column = unique_key ), 0)
    END REPEAT;
    RETURN unique_key;
END