How to create a column in a table from a Select

Question

I have the table users and the table posts, need to copy the same information from a column of table users to a column of table posts, for example I have this code:

SELECT posts.*, users.username
FROM posts
  inner join users on (posts.user_id = users.id )

which brings me a query of exactly what I need, but in my php project I need this column to be created, to be able to visualize it by my html.

One more example to help in understanding my question, if I run the code:

ALTER TABLE articles ADD COLUMN user_id INT(11);

he can create exactly what I need, but with the field "id" which is in the users table, but only with the "id", if I make the same code with the field I need, which is the username: "user_username", returns me null.