public NumberExpression<Double> avg() {
        if (avg == null) {
            avg = Expressions.numberOperation(Double.class, Ops.AggOps.AVG_AGG, mixin);
        }
        return avg;
    }
Create a {@code avg(this)} expression

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

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