Turn existing column into a primary key

Question

create table emp
 (
 emp_id int,
 emp_name varchar(40) not null,
 emp_address varchar(35) not null,
 )

Now I need to add primary key to emp_id . I tried using alter query

alter table emp add constraint   emp_id  primary key

its showing error as

Table level constraint does not specify column list, table 'emp'.

Answer 1

ALTER TABLE emp
ADD PRIMARY KEY (emp_id)

OR

ALTER TABLE emp
ADD CONSTRAINT pk_empID PRIMARY KEY (emp_id)