MathException: Divide by Zero

What does this error mean?

Apex throws System.MathException when integer or decimal division by zero is attempted. This is common in reporting and rollup logic where the divisor is derived from a COUNT() or SUM() aggregate that returns zero for a particular group — for example, calculating a win rate where no opportunities have been closed in a given period.

Common Causes

1. COUNT() aggregate returns zero for a group

A win-rate formula like wonCount / totalCount where totalCount is zero for a particular owner or period — data-dependent and difficult to catch in testing.

2. SUM() returns zero or null

Dividing by a SUM() result that is null (no matching records) or zero (all values are 0) without null/zero checking.

How to Fix It

Solution 1: Guard the divisor with a null/zero check

// ❌ BAD — throws if totalCount is 0
Decimal winRate = wonCount / totalCount;

// ✅ GOOD — guard against zero
Decimal winRate = (totalCount != null && totalCount != 0)
    ? (wonCount / totalCount) * 100
    : 0;

// ✅ Pattern for aggregate result
for (AggregateResult ar : [
    SELECT OwnerId, COUNT() total, SUM(Amount) revenue
    FROM Opportunity
    GROUP BY OwnerId
]) {{
    Integer total   = (Integer) ar.get('total');
    Decimal revenue = (Decimal) ar.get('revenue');

    Decimal avg = (total != null && total > 0)
        ? revenue / total
        : 0;
    System.debug('Avg deal: ' + avg);
}}