About TAB and DUAL

DUAL is a SYS owned table. It is normally used to return values from stored functions, sequence values, and so on. It is recommended not to drop or perform any DML operations against a DUAL table. The following is an example of fetching the current date using the DUAL table:

SQL> SELECT sysdate FROM dual;
SYSDATE
---------
10-MAY-12
SQL>

TAB is a SYS owned view which is used to list tables and views in a table. The following is a sample query to list all tables/views in the current schema:

SQL> SELECT * FROM tab;