SQL & PL/SQL

n an application, I intend on truncating and inserting on an Oracle 12c database, but have found this issue with an IDENTITY column. Even though the INSERT... SELECT statement works on most SELECT uses I have tried, when this statement also has a GROUP BY clause, it fails to work, issuing a "ORA-00979: not a GROUP BY expression" complaint. Below is some example code:

create table aux ( owner_name varchar2(20), pet varchar2(20) );  
insert into aux values ('Scott', 'dog'); 
insert into aux values ('Mike', 'dog'); 
insert into aux values ('Mike', 'cat'); 
insert into aux values ('John', 'turtle');  


create table T1 (
  id number generated always as identity,
  owner_name varchar2(20),
  pet_count number ); 
select owner_name, count(*) as pet_count from aux group by owner_name; -- works just fine
insert into T1 (owner_name, pet_count) select owner_name, count(*) as pet_count from aux group by owner_name; -- doesn't work

The select statement works by itself, but fails as a INSERT ... SELECT statement.

Appreciate the help!