public static <T extends Number> WindowOver<T> stddevPop(Expression<T> expr) {
        return new WindowOver<T>(expr.getType(), SQLOps.STDDEVPOP, expr);
    }
returns the population standard deviation and returns the square root of the population variance.

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