NVL and no rows returned

Question

Hi,
i found a little error in coded in an SQL-Construct equivalent to this one:
SELECT NVL(dummy, 'A') from dual where 1=2
This will return nothing instead of the expected 'A', because in my understanding no rows are returned and though no NULL could be replaced.
BUT, at least surprising for me,
SELECT NVL(
(SELECT NVL(dummy, 'A') from dual where 1=2), 'B')
from dual
will return 'B'.
What would be a good explanation of this behaviour?
I found nothing in the documentation on this.
Strange enough for me, in the net often other 'workarounds" are proposed, like
SELECT * from(
SELECT NVL(dummy, 'A') from dual where 1=2
UNION ALL
SELECT 'A' from dual)
where
rownum = 1
or with the use of EXISTS.
Are there any drawbacks of the simple NVL-wrapper?
Regards
Chris
Edited by: chris227 on 11.01.2010 01:43

Answer 1

What would you say is the resultset of 'select NULL from dual' ?It's aset wich contains one element: NULL
and, then,
SQL> select nvl((select * from empty_table),'B') from dual;select * from empty_table returns an empty set, called NULL. hence NVL evaluates to 'B'
NVL((
B
SQL> select * from empty_table;
no rows selectedThat means, there are no elements in the result set at all. You wanted to see all those elements.
That was what i wanted to say with the naiv words resultset and columns.
set = resulstset
element = columns (projection)
Regards
chris