Select Query statement

Hi to All,

Below is a sample mssql query given to me, now I am wondering if can I add a where statement to it? because right now I am trying to add a where clause in it and I'm getting an error.

The mssql query statement:
select substring(username, 1, charindex(',',username, 2) - 1) from tbllessons

Right now if you see the title for my question is different I am sorry as to I cannot think the right question title for it.

Thanks