How to create identity field in MySQL without using auto_increment column?
You have to use UUID() function that guarantee that returned string is unique not only within your table or database but all over the world. 

http://dev.mysql.com/doc/refman/5.5/en/miscellaneous-functions.html#function_uuid


  A UUID is designed as a number that is globally unique in space and
  time. Two calls to UUID() are expected to generate two different
  values, even if these calls are performed on two separate computers
  that are not connected to each other.


Also there is UUID_SHORT() function that returns 64-bit INT instead of string, that is way faster for indexing and search. 

http://dev.mysql.com/doc/refman/5.5/en/miscellaneous-functions.html#function_uuid-short