public static <T extends Number> WindowOver<T> stddev(Expression<T> expr) {
        return new WindowOver<T>(expr.getType(), SQLOps.STDDEV, expr);
    }
returns the sample standard deviation of expr, a set of numbers.

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