When there is no rows in select statement And query return "null" value in zabbix_server.log, there are very suspicious logs.
Probably zabbix goes in memory area reserved by any other function.
Especially look on 16316:20120731 which return absolutely nonsense information got from some config "memory area".

16315:20120731:122403.447 item [GGSN40990B:db.odbc.select[EMSPort-22-1-RX]] became not supported: Received value [@?] is not suitable for value type [Numeric (unsigned)] and data type [Decimal]
16317:20120731:122616.205 item [GGSN40990B:db.odbc.select[sess-txbytes]] became not supported: Received value [?] is not suitable for value type [Numeric (unsigned)] and data type [Decimal]
16318:20120731:122758.427 item [rmsc-h1:db.odbc.select[tx-in-unknown-files]] became not supported: Received value [p?] is not suitable for value type [Numeric (unsigned)] and data type [Decimal]
16315:20120731:122808.708 item [rmsc-h1:db.odbc.select[tx-in-text-files]] became not supported: Received value [??] is not suitable for value type [Numeric (unsigned)] and data type [Decimal]
16316:20120731:122813.867 item [rmsc-h1:db.odbc.select[tx-out-image-files]] became not supported: Received value [ymax_itemid=null,flags=4 where graphid=5162;update graph_discovery set name='Trafic on interface

' w
16316:20120731:122953.427 item [GGSN40990B:db.odbc.select[EMSPort-22-3-TX]] became not supported: Received value [P?] is not suitable for value type [Numeric (unsigned)] and data type [Decimal]
16316:20120731:123008.851 item [GGSN40990B:db.odbc.select[EMSPort-22-1-TX]] became not supported: Received value [] is not suitable for value type [Numeric (unsigned)] and data type [Decimal]
16317:20120731:123304.305 item [GGSN40990B:db.odbc.select[EMSPort-27-3-RX]] became not supported: Received value [0?] is not suitable for value type [Numeric (unsigned)] and data type [Decimal]
16317:20120731:123330.411 item [rmsc-h2:db.odbc.select[tx-pass-through-files]] became not supported: Received value [?S] is not suitable for value type [Numeric (unsigned)] and data type [Decimal]

In the same time:
mysql> select * from events where device='rmsc-h1';
Empty set (0.00 sec)

mysql> select * from events where device='GGSN40990B';
Empty set (0.00 sec)

My odbc item for example:

—
DSN=MySQLP
sql=select round(avg(params.param2)) from events LEFT JOIN params ON events.id=params.events_id where itemname = "EMSPort" and device='{$NAZWA}' and itemvalue='223' and eventtime > UNIX_TIMESTAMP()-{$AVG2XTIME};
—

Best Regards,
Grzegorz