Mysql-about the problem that Mysql cannot be inserted after setting a trigger

I have set a trigger in Mysql. if the user has not modified the user name, the uid can be used as the default user name: entercodehere trigger, so you cannot modify your own table... DELIMITER | createtriggerdefault_nameafterinsertonuserforeachrowbeginupdateusersetnew.user_namene1_uid; END | a data entry is inserted and the following error occurs: can & #39; tupdate or another mysql trigger

I have set a trigger in Mysql to use uid as the default user name without modifying the user name:

Enter code here
/// The trigger cannot modify its own table...

DELIMITER |

Create trigger default_name after insert on user
FOR EACH ROW BEGIN
Update user set NEW. user_name = NEW. uid;

END
|
Then, a piece of data is inserted and the following error occurs:
Can't update ,,
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