public static <T extends Number> WindowOver<T> sum(Expression<T> expr) {
        return new WindowOver<T>(expr.getType(), Ops.AggOps.SUM_AGG, expr);
    }
Start a window function expression

@param expr expression
@return sum(expr)
querydsl-sql/src/main/java/com/querydsl/sql/SQLExpressions.java