ランダムな文字列の生成

SUBSTRING('abc,,789',)

 SELECT CONCAT(
           SUBSTRING('abcdefghijklmnopqrstuvwxyz0123456789', CEIL(RAND() * 36), 1),
           SUBSTRING('abcdefghijklmnopqrstuvwxyz0123456789', CEIL(RAND() * 36), 1),
           SUBSTRING('abcdefghijklmnopqrstuvwxyz0123456789', CEIL(RAND() * 36), 1),
           SUBSTRING('abcdefghijklmnopqrstuvwxyz0123456789', CEIL(RAND() * 36), 1)
       );

MD5()

 SELECT SUBSTRING(MD5(RAND()), 1, 4);

MD5()は32桁の16進数を返す。https://dev.mysql.com/doc/refman/5.5/en/encryption-functions.html#function_md5

UUID()

 SELECT UUID();

http://dev.mysql.com/doc/refman/5.6/en/miscellaneous-functions.html#function_uuid