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
