Can null values be sorted low?

The problem...
 From the table below I want to select distinct clus_id and the data in
3 db columns, even if that data is null.

When there are multiple contigs for a clus_id I want to select the row
where 1st, the most db columns have a value, 2nd the total value of the
db columns is highest.

So for
LRC00006 I want contig 3,
LRC00010 I want contig 4, because it has 3 db values (even though contig
2 would total more)
LRC00001 I want contig 2, because it would total more if the null value
was treated as zero in the calculation
and for the rest I want contig 1.

Any hints on writing such a query would be much appreciated.

lumbribase=# select clus_id,contig,db0,db1,db4,(db0+db1+db4) as total
from venn order by clus_id,total DESC;
 clus_id  | contig | db0  | db1  | db4  | total
----------+--------+------+------+------+-------
 LRC00001 |      1 | 77.4 |      | 63.9 |
 LRC00001 |      2 | 77.4 |      | 82.7 |
 LRC00002 |      1 |  325 |  343 |  313 |   981
 LRC00003 |      1 |      |      |      |
 LRC00004 |      1 |      |      |      |
 LRC00005 |      1 |  294 |  294 |  116 |   704
 LRC00006 |      1 |  100 | 72.8 |      |
 LRC00006 |      3 |  120 |  122 | 63.9 | 305.9
 LRC00006 |      2 |  117 |  112 | 58.5 | 287.5
 LRC00007 |      1 |  178 |      |      |
 LRC00008 |      1 |      |      |      |
 LRC00009 |      1 |  416 |      |      |
 LRC00010 |      2 |  324 |  167 |     |
 LRC00010 |      4 |  146 |  168 |  172 |   486
 LRC00010 |      1 |  146 |  166 |  171 |   483
 LRC00010 |      3 |  145 |  160 |  159 |   464
 LRC00011 |      1 |  179 |  100 | 95.1 | 374.1
 LRC00012 |      1 |  639 |  639 |  633 |  1911
 LRC00012 |      3 |  505 |  509 |  508 |  1522
 LRC00012 |      2 |  390 |  391 |  392 |  1173



--
Ann

"In a world without walls and fences - who needs Windows and Gates ?"
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