Some Important operation in DUAL table

1) SELECT greatest(12,54,2,75,142) from dual;
2) SELECT level as ID
FROM DUAL
CONNECT BY level < 101
3) select 5 * 6 from dual
SELECT USER FROM DUAL
SELECT COUNT(*) FROM DUAL;
INSERT INTO DUAL VALUES ('X');
4) select 'test' from dual;
5) select object_name, owner, object_type from dba_objects where object_name = 'DUAL';