Mathematical operations with NULL values

Hi,

given a table with some data, e.g. some monthly measures. Some of the
measures are missing though.

id m1 m2 m3 m4 m5 .... m12
----------------------------------------------

1 23 45 66 76 76 .... 12
2 76 NULL 77 88 77 ... 89
3 67 87 98 NULL 78 ... NULL

I would like the calculate the yearly average of each row, something
like ((m1+m2+m3+m4+m5+...m12)/12). This would work if I had all montly
values for one year. In the case of at least one NULL value involved, I
would get NULL as result.

So instead of dividing each year by 12, I would have to divide by the
number of measures available in each row.

Could someone point me to the correct SQL syntax for doing this.

Thanks a lot
alex.

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Departement of Geography and Regional Research
University of Vienna
Cartography and GIS
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Virtual Map Forum: http://www.gis.univie.ac.at/vmf
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