Oracle 18c

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create table t1
(
   id integer primary key, 
   c1 varchar(10)
)
insert into t1 (id, c1) values (1, 'One')
1 rows affected

alter table t1
   add (c2 number not null)
ORA-01758: table must be empty to add mandatory (NOT NULL) column

alter table t1
   add (c2 number default 42 not null)
select *
from t1


ID
C1
C2


1
One
42