ORA-00932 while using nvl with trunc function

Question

What happens when trunc is executed on null?
I am executing the following query:
"select nvl(trunc(null),sysdate) from dual"
and this throws "ORA-00932: inconsistent datatypes: expected NUMBER got DATE, error at Line:1 Column:23".
Whereas "select nvl(null,sysdate) from dual" returns sysdate correctly.
Also "select trunc(null) from dual" returns null only. So when the returned null is passed through nvl why am I getting exception?
This is happening in ver 9.2.0.5.0 and 10.2.0.2.0
There is another observation, which is an issue that we have found in our code, and while trying to fix that we saw the earlier observation.
SELECT * FROM orgs, dual where
trunc(SYSDATE) between trunc(orgs.effective_start_date) and nvl(trunc(orgs.effective_end_date),trunc(sysdate))
Here effective start date and effective end date for orgs are null for all records.
When we run this query on ver 9.2.0.5.0, this runs without any exception. But when we run this query on ver 10.2.0.2.0, we get the same exception, "ORA-00932: inconsistent datatypes: expected NUMBER got DATE, error at Line:2 Column:95".
The join with dual is fake, in actual scenario we have join with other tables, but since we are able to replicate with dual, removed all other details to keep this simple.
Now if we remove the join with dual, the query works fine in both the env.
SELECT * FROM orgs where
trunc(SYSDATE) between trunc(orgs.effective_start_date) and nvl(trunc(orgs.effective_end_date),trunc(sysdate))

Answer 1

What happens when trunc is executed on null?
I am executing the following query:
"select nvl(trunc(null),sysdate) from dual"
and this throws "ORA-00932: inconsistent datatypes:
expected NUMBER got DATE, error at Line:1
Column:23".
Whereas "select nvl(null,sysdate) from dual" returns
sysdate correctly.
Also "select trunc(null) from dual" returns null
only. So when the returned null is passed through nvl
why am I getting exception?
This is happening in ver 9.2.0.5.0 and 10.2.0.2.0The first parameter to NVL is determining the expected datatype of the returned column, with the trunc function defaulting that to NUMBER because it's parameter is NULL. The second parameter to NVL needs to match that datatype which it doesn't because it is a date.
SQL> select nvl(trunc(sysdate), sysdate) as mydate from dual;
MYDATE
26/05/2006 00:00:00
SQL> select nvl(trunc(null), sysdate) as mydate from dual;
select nvl(trunc(null), sysdate) as mydate from dual
ERROR at line 1:
ORA-00932: inconsistent datatypes: expected NUMBER got DATE
SQL> select nvl(trunc(123), sysdate) as mydate from dual;
select nvl(trunc(123), sysdate) as mydate from dual
ERROR at line 1:
ORA-00932: inconsistent datatypes: expected NUMBER got DATE
SQL>