If particular condition is not satisfied in case how to say that to be zero

select sal, case when sal < 2000 then 'category 1' 
                 when sal < 3000 then 'category 2' 
                 when sal < 4000 then 'category 3' 
                 else 'category 4' 
            end 
from emp; 
In the query above how to say Category 1  = 0 when there are no employees with salary less than 2000