How to select rows based on other tables - MySql
Is this what you're looking for?

SELECT Whatever FROM UserTable
WHERE UserTable.username NOT IN 
(
  SELECT CheckTable.user_id FROM CheckTable
);


If this is not suitable, please expand. Possibly with table definitions - (SHOW CREATE TABLE My_Table\G) and some sample data - (INSERT INTO My_Table VALUES(....)), and finally, the result you want with the logic you used to obtain it.

With further research, you might also find this interesting - and also this. As a generic response, substituting NOT EXISTS for NOT IN might be the way to go - YMMV.