public NumberExpression<T> sum() {
        if (sum == null) {
            sum = Expressions.numberOperation(getType(), Ops.AggOps.SUM_AGG, mixin);
        }
        return sum;
    }
Create a {@code sum(this)} expression

<p>Get the sum of this expression (aggregation)</p>

@return sum(this)
querydsl-core/src/main/java/com/querydsl/core/types/dsl/NumberExpression.java