if you want to remove the column sal from emp permanently there are two other ways
1) Alter table emp drop column sal;
2) alter table emp set unused column sal;
This option will keep the column sal but then it is as good as it is not present. The only operation allowed on column after setting it as unused is to DROP it using first choice.
as given by earlier reply
3) well if you don't want to drop the column sal but you just want to delete the values in the column sal then you should be using
Update emp set sal=NULL