This query falsely returns 0.0617:
SELECT SUM(TIME)/3600 FROM idletimehistory WHERE idle =1 and TIMESTAMP >= NOW() - INTERVAL 30 DAY AND vehicleid IN (select id from vehicles where fleetid =2000)
Using MYSQL server on the command line it returns NULL, as it should.
Also Heidi 9.5.0.5196 gives NULL as does a recent install of DBever.