在使用 MySQL 數據庫的過程中，有時我們需要生成一些隨機數來作為唯一的標識符。MySQL 中可以使用函數來生成隨機數，當然我們可以使用自己的算法來進行生成。下面我們來看一下如何使用 MySQL 來生成一個18位的隨機數。

DELIMITER $$
CREATE FUNCTION rand18() RETURNS CHAR(18)
BEGIN
DECLARE chars CHAR(62) DEFAULT '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz'; 
DECLARE rNum1 INT DEFAULT 0;
DECLARE rNum2 INT DEFAULT 0;
DECLARE rNum3 INT DEFAULT 0;
DECLARE rNum4 INT DEFAULT 0;
DECLARE rNum5 INT DEFAULT 0;
DECLARE rNum6 INT DEFAULT 0;
DECLARE rNum7 INT DEFAULT 0;
DECLARE rNum8 INT DEFAULT 0;
DECLARE rNum9 INT DEFAULT 0;
DECLARE rNum10 INT DEFAULT 0;
DECLARE rNum11 INT DEFAULT 0;
DECLARE rNum12 INT DEFAULT 0;
DECLARE rNum13 INT DEFAULT 0;
DECLARE rNum14 INT DEFAULT 0;
DECLARE rNum15 INT DEFAULT 0;
DECLARE rNum16 INT DEFAULT 0;
DECLARE rNum17 INT DEFAULT 0;
DECLARE rNum18 INT DEFAULT 0;
SET rNum1 = FLOOR(RAND() * 62) + 1;
SET rNum2 = FLOOR(RAND() * 62) + 1;
SET rNum3 = FLOOR(RAND() * 62) + 1;
SET rNum4 = FLOOR(RAND() * 62) + 1;
SET rNum5 = FLOOR(RAND() * 62) + 1;
SET rNum6 = FLOOR(RAND() * 62) + 1;
SET rNum7 = FLOOR(RAND() * 62) + 1;
SET rNum8 = FLOOR(RAND() * 62) + 1;
SET rNum9 = FLOOR(RAND() * 62) + 1;
SET rNum10 = FLOOR(RAND() * 62) + 1;
SET rNum11 = FLOOR(RAND() * 62) + 1;
SET rNum12 = FLOOR(RAND() * 62) + 1;
SET rNum13 = FLOOR(RAND() * 62) + 1;
SET rNum14 = FLOOR(RAND() * 62) + 1;
SET rNum15 = FLOOR(RAND() * 62) + 1;
SET rNum16 = FLOOR(RAND() * 62) + 1;
SET rNum17 = FLOOR(RAND() * 62) + 1;
SET rNum18 = FLOOR(RAND() * 62) + 1;
RETURN CONCAT(SUBSTRING(chars, rNum1, 1), SUBSTRING(chars, rNum2, 1), 
SUBSTRING(chars, rNum3, 1), SUBSTRING(chars, rNum4, 1), 
SUBSTRING(chars, rNum5, 1), SUBSTRING(chars, rNum6, 1), 
SUBSTRING(chars, rNum7, 1), SUBSTRING(chars, rNum8, 1), 
SUBSTRING(chars, rNum9, 1), SUBSTRING(chars, rNum10, 1), 
SUBSTRING(chars, rNum11, 1), SUBSTRING(chars, rNum12, 1), 
SUBSTRING(chars, rNum13, 1), SUBSTRING(chars, rNum14, 1), 
SUBSTRING(chars, rNum15, 1), SUBSTRING(chars, rNum16, 1), 
SUBSTRING(chars, rNum17, 1), SUBSTRING(chars, rNum18, 1));
END $$
DELIMITER ;

這段代碼利用了 MySQL 中的一個自定義函數，生成了一串由數字和大小寫字母組成的18位隨機數。