SQL> create table tbl as select dname from dept;

Table created.

SQL> alter table tbl add id int generated always as identity primary key not null;
alter table tbl add id int generated always as identity primary key not null
            *
ERROR at line 1:
ORA-01758: table must be empty to add mandatory (NOT NULL) column


SQL> alter table tbl add id int generated by default on null as identity;

Table altered.

SQL> update tbl set id = rownum;

4 rows updated.

SQL> alter table tbl modify id generated always as identity(start with limit value) not null;

Table altered.

SQL> alter table tbl add constraint pk_tbl primary key(id);

Table altered.

SQL> select * from tbl;

DNAME                  ID
-------------- ----------
ACCOUNTING              1
RESEARCH                2
SALES                   3
OPERATIONS              4

SQL> insert into tbl(dname) values('SHIPPING');

1 row created.

SQL> select * from tbl;

DNAME                  ID
-------------- ----------
ACCOUNTING              1
RESEARCH                2
SALES                   3
OPERATIONS              4
SHIPPING                5

SQL>