I seem to have hit a bug (I just reported it) and now I'm using an ugly hack to get around it but I was wondering if there's a better way to avoid division by zero errors. The bug is that MonetDB seems to evaluate both branches of a CASE regardless of whether the branch will be taken or not: sql>SELECT more>CASE SUM(payout_units_cpc) more>WHEN 0 THEN 0 more>ELSE 1 more>END AS avg_cost_cpc more>FROM daily.stats_2009_week_31; SELECT +--------------+ | avg_cost_cpc | +==============+ | 0 | +--------------+ 1 tuple Timer 27.990 msec 1 rows sql>SELECT more>CASE SUM(payout_units_cpc) more>WHEN 0 THEN 0 more>ELSE cast(SUM(payout_units_cpc * payout_cpc) as numeric(12,4)) / SUM(payout_units_cpc) more>END AS avg_cost_cpc more>FROM daily.stats_2009_week_31; !MALException:calc./:Illegal argument Division by zero 0 tuples Timer 16.415 msec 0 rows So what I did was I changed my query to: SELECT CASE SUM(payout_units_cpc) WHEN 0 THEN 0 ELSE cast(SUM(payout_units_cpc * payout_cpc) as numeric(12,4)) / (SUM(payout_units_cpc) + 0.000001) END AS avg_cost_cpc FROM daily.stats_2009_week_31 WHERE date_added >= 1249099200 AND date_added <= 1249185599 Because of the decimal precision it won't affect my numbers but it feels wrong.