Example
This is my preferred approach. Nothing related to performance just a personal choice. It makes the code to look clean. You can see the resulting column names easily instead of scrolling the code if you have a big expression.
CREATE TABLE AliasNameDemo (id INT,firstname VARCHAR(20),lastname VARCHAR(20))
INSERT INTO AliasNameDemo
VALUES (1,'MyFirstName','MyLastName')
SELECT FullName = FirstName +' '+ LastName
FROM AliasNameDemo
