How to get a unique RFID phone?

Question

MySQL

The essence of the problem
RFID-RC522
Adruino reader
The RFID
library normally reads uid cards, but the phone is different every time.
Am I reading the wrong code? Or do I not understand something?
mfrc522.uid.uidByte
code from the example.

Answer 1

SELECT `u`.`id`, `u`.`name`
  FROM `article_subject` AS `as`
  JOIN `user_subject` AS `us` 
    ON `as`.`article_id` = :article_id AND `us`.`subject_id` = `as`.`subject_id`
  JOIN `users` AS `u` 
    ON `u`.`id` = `us`.`user_id`

Answer 2

phone is different every time

That's the way it should be.