This is my table. I need to add a null constraint using alter table.

alter table gradstudent
add constraint gradstudent_sfname_nn not null(s_fname);

alter table gradstudent
add constraint gradstudent_slname_nn not null(s_lname);

What am I doing wrong? Is it possible to put it in one statement?

Name Null? Type
----------------------------------------- -------- ----------------------------
S_ID NUMBER(3)
S_LAST VARCHAR2(10)
S_FIRST VARCHAR2(10)
S_MI CHAR(1)
S_ADD VARCHAR2(25)