What means ".... FROM DUAL" in SQL statements?

Question

Occasionally I find in different SQL statement the appedix "....from dual" e.g.
SELECT blahblah FROM DUAL;
It seems to me that this is a special built-in table.
What does that mean exactely?
Peter

Answer 1

I guess 'FROM' clause is mandatory in Oracle. Hence,
there must always be a table from which we are
selecting values. So, for selecting some arbitary
values which are not coming from any DB Table, we use
this dummy table 'Dual'.
Like
select substr('sometext',5) from dual;
select sysdate from dual ;
etc.
Hope this clarifies.This explanation might be misleading.
The FROM clause is of cause mandatory for a SELECT statement (I think any SQL DB system, not only Oracle). So whenever you need to write a SELECT you might need a dual table.
If you are in PL/SQL you can use most functions without doing a SELECT. Like
a:= substr('sometext',5);
myDate := sysdate;For some things we need to do a select. If we don't have a matching table, then the dual table is handy to return only one row.
example
SELECT mySequence.nextval FROM DUAL;When we are using some tools, like SQL*Plus or T.O.A.D. our typical path to get information returned to the gui is a select. This would be a case were your examples could be used. But you could also do it like this: execute dbms_output.put_line(to_char(sysdate)); After switching the serveroutput on.