Connected to Oracle Database 18c Express Edition Release 18.0.0.0.0 
Connected as user1@DBOFFICEXE

SQL> create table t1(n number);

Table created


SQL> create table t1(n number);
create table t1(n number)

ORA-00955: name is already used by an existing object

SQL> create view t1 as select * from user_tables;
create view t1 as select * from user_tables

ORA-00955: name is already used by an existing object

SQL> alter table t1 add d date;

Table altered


SQL> alter table t1 add n number;;
alter table t1 add n number;

ORA-01735: invalid ALTER TABLE option

SQL> alter table t1 add n number;
alter table t1 add n number

ORA-01430: column being added already exists in table

SQL> dtop table t2;
dtop table t2

ORA-00900: invalid SQL statement

SQL> drop table t2;
drop table t2

ORA-00942: table or view does not exist

SQL> drop table t1;

Table dropped


SQL>