SUM() of 1 and NULL is 1 ?

[C.R.Vegelin]

Hi List,

I need to SUM() on months from a table like:
CREATE TABLE `data`
( `Jan` float default NULL,
...
) ENGINE=MyISAM; # V 5.0.15

Months may have NULL values, like:
INSERT INTO data (Jan) VALUES (1), (NULL);

However, when I use SELECT SUM(Jan) AS Jan,
the returned value is 1 in stead of NULL.
How to get a result NULL when in such a case ?

TIA, Cor