: can anybody help me to explain what is DUAL in oracle ?
: is it the system table created with each and every Database? what are the
: other files created with it? 
: i mean system tables and their use.
: leo

Leo,

DUAL is an interesting table - it contains ONE column and ONE record.
Oracle has created this since it makes some calculations more convenient.

For example, you can use it for math:

SELECT (319/212)+10 FROM DUAL;

You can use it to increment sequences:

SELECT employee_seq.NEXTVAL FROM DUAL;

You can use it to play around or test some SQL:

SELECT CHR(70) FROM DUAL;

As for all of the other tables, there are too many to mention. I suggest getting
either the Platinum (www.platinum.com) or TUSC (www.tusc.com) poster detailing
the system tables. Otherwise these are well documented in the Oracle Docs and
several books.

Best regards,

-Ari Kaplan

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