SQL count distinct与NULL

$sh@ORCL> select * from tt2;

    TT2_ID TT2_NAME   TT2_DATE
---------- ---------- ---------
         1 1          23-DEC-13
         2            23-DEC-13
         3            23-DEC-13
         4 1          23-DEC-13
$sh@ORCL> SELECT count(tt2_name) FROM tt2;

COUNT(TT2_NAME)
---------------
              2

$sh@ORCL> select distinct tt2_name from tt2;

TT2_NAME
----------
1

$sh@ORCL> select sum(tt2_name) from tt2;

SUM(TT2_NAME)
-------------
            2 
$sh@ORCL> select avg(tt2_name) from tt2;

***G(TT2_NAME)
-------------
            1