sql: DUAL

FROM <<Oracle.Database.11g.SQL>>

dual is a table that contains a single row. The following output from the DESCRIBE command shows the structure of the dual table, along with a query that retrieves the row from the dual table:

DESCRIBE dual
Name Null? Type
----------------------------------------- -------- -----------
DUMMY VARCHAR2(1)

SELECT *
FROM dual;
D
-
X

Notice the dual table has one VARCHAR2 column named dummy and contains a single row with the value X.

-------------

SELECT TO_DATE('02-AUG-2007') - 3
FROM dual;
TO_DATE('
---------
30-JUL-07

You can also subtract one date from another, yielding the number of days between the two dates. The following example subtracts July 25, 2007, from August 2, 2007:
SELECT TO_DATE('02-AUG-2007') - TO_DATE('25-JUL-2007')
FROM dual;
TO_DATE('02-AUG-2007')-TO_DATE('25-JUL-2007')
---------------------------------------------
8

NOTE:

TO_DATE() is a function that converts a string to a date.

SELECT 10 * 12 / 3 – 1
FROM dual;
10*12/3-1
----------
39

SELECT TO_DATE('02-AUG-2007') - 3
FROM dual;
TO_DATE('
---------
30-JUL-07

　　

select 1 from dual where null=null; 
1
select 1 from dual where null='';
1
select 1 from dual where ''='';
1

___________________________________

select 1 from dual where null is null;  
1
---
1
select 1 from dual where nvl(null,0)=nvl(null,0); 
1
---
1
select user from dual; 
USER
---
FAIRPT
select sysdate from dual; 
SYSDATE
---
24-NOV-15