Help understand difference in sql query

Question

Select user_name [User Name], first_name [First Name], last_name [Last Name] 
    From tab_user
    ORDER  BY user_name

Select user_name [User Name], first_name [First Name], last_name [Last Name] 
    From tab_user
    ORDER  BY User Name

Above are the two queries,

1. Is there any difference because of the user_name used instead of User Name
2. Is there something that should be taken care/worried when using something like this.

Answer 1

the second query shouldn't work should it? you would need to

order by [User Name]

Answer 2

Yes, it has to be user_name and not user uame. Space is not allowed.

Answer 3

Your second query won't work. It needs to be this:

ORDER BY [User Name]

Once this is fixed there is no difference in the results. The syntax SELECT User_Name [User Name] creates an alias.