How to name a constraint which was unnamed during the table creation?
When working with Oracle, you can find the constraint names (and other details about constraints) by querying USER_CONSTRAINTS.

Example (Oracle 12c)

-- table with PK and NN constraints
create table test ( 
  id number 
    generated always as identity start with 2000 
    primary key
, description varchar2( 100 ) not null
);

-- populate the table (optional)
insert into test ( description )
select 'description_' || to_char( level )
from dual
connect by level &lt;= 5 ;


select * from test ;
ID    DESCRIPTION    
2000  description_1  
2001  description_2  
2002  description_3  
2003  description_4  
2004  description_5 


Find constraints

select table_name, constraint_name, constraint_type, search_condition
from user_constraints
where table_name = 'TEST' ;  -- table_name: use upper case!

TABLE_NAME  CONSTRAINT_NAME  CONSTRAINT_TYPE  SEARCH_CONDITION           
TEST        SYS_C0021091     P                NULL                       
TEST        SYS_C0021089     C                "ID" IS NOT NULL           
TEST        SYS_C0021090     C                "DESCRIPTION" IS NOT NULL 


Rename the constraints

alter table test
rename constraint SYS_C0021091 to test_pk ;

alter table test
rename constraint SYS_C0021089 to id_nn ;

alter table test
rename  constraint SYS_C0021090 to description_nn ;


Do the constraints have new names now? Yes.

select table_name, constraint_name, constraint_type, search_condition
from user_constraints
where table_name = 'TEST' ;

TABLE_NAME  CONSTRAINT_NAME  CONSTRAINT_TYPE  SEARCH_CONDITION           
TEST        TEST_PK          P                NULL                       
TEST        ID_NN            C                "ID" IS NOT NULL           
TEST        DESCRIPTION_NN   C                "DESCRIPTION" IS NOT NULL 


Data still intact? Yes.

SQL&gt; select * from test;
ID    DESCRIPTION    
2000  description_1  
2001  description_2  
2002  description_3  
2003  description_4  
2004  description_5