it's not NULL, then what is it?

Hi Everybody,

I have a table called gallo.sds_seq_reg_shw,
which is like:

canon=# \d gallo.sds_seq_reg_shw
Table "gallo.sds_seq_reg_shw"
Column | Type | Modifiers
------------------+---------+-----------
name | text |
response | text |
n | integer |
source | text |
test | text |
ref | text |
value | real |
pvalue.term | real |
stars.term | text |
gtclass.test | text |
fclass.test | text |
gtclass.ref | text |
fclass.ref | text |
markerid | integer |
maf | real |
chromosome | text |
physicalposition | integer |
id | text |
ctrast | text |

I am intereseted in the column maf (which is real):

canon=# select maf from gallo.sds_seq_reg_shw
canon-# order by maf asc;
maf
-------------
0.000659631
0.000659631
0.000659631
0.000659631
.
(trunacated for the interest of breivity)
.

Another way to look at this column is:

canon=# select maf from gallo.sds_seq_reg_shw
canon-# order by maf desc;
maf
-------------



.
(trunacated for the interest of breivity)
.

These rows shown are blanks, as far as I can tell.
But...

canon=# select count(maf) from gallo.sds_seq_reg_shw;
count
-------
67284
(1 row)

canon=# select count(maf) from gallo.sds_seq_reg_shw
canon-# where maf ISNULL;
count
-------
0
(1 row)

canon=#
canon=# select count(maf) from gallo.sds_seq_reg_shw
canon-# where maf NOTNULL;
count
-------
67284
(1 row)

My confusion is that if they are real and not null,
what are they? How would I construct a query to do
something like:

select count(maf)
from gallo.sds_seq_reg_shw
where maf ISBLANK;

Thank you in advance.

Regards,

Tena Sakai
tsakai(at)gallo(dot)ucsf(dot)edu