how to add primary key on existing table in sql server ?

how to add primary key on existing table in sql server ?

    Please refer the below example syntex :

  table my_table( m_col1 int ,m_col2 varchar(20),m_col3
 alter
 alter table my_table add constraint pk_m_no primary key (m_col1); table my_table alter column m_col1 int not null;datetime);
create