Some Oracle commands

Renaming a table:
alter table oldname rename to newname;

Renaming a column:
alter table sometable rename column oldname to newname;

Return only a specified number of rows:
select * from mytable where rownum;

Make a backup copy of a table:
create table mytable_backup as select * from mytable;

Get a list of tables:
select * from cat;

Get the create command for a table (showing the primary key):
select dbms_metadata.get_ddl('TABLE',upper('MYTABLE')) from dual;

Change a user password:
alter user user_name identified by new_password;

Make things print to screen nicer:
set pagesize 0
set long 999999