SQL string manipulation [Get all text left of '(']

You can;

LEFT(SourceOfBooking, CHARINDEX(' (', SourceOfBooking + ' (') - 1)

(Remove + ' (' if needed; it allows for rows with no  (

I think you've just put a wrong character

case
    when CHARINDEX('(', SourceOfBooking) > 0 then
        rtrim(left(SourceOfBooking, CHARINDEX('(', SourceOfBooking) - 1))
    else
        SourceOfBooking
end