"ORDER BY(SELECT NULL)"是什么意思？

Question

以下SQL来自Itzik Ben-Gan,用于生成数字表.这个order by (select null)部分是什么意思？谢谢.

DECLARE @number_of_numbers INT;
SELECT @number_of_numbers = 100000;

WITH    a AS ( SELECT   1 AS i
               UNION ALL
               SELECT   1
             ),
        b AS ( SELECT   1 AS i
               FROM     a AS x ,
                        a AS y
             ),
        c AS ( SELECT   1 AS i
               FROM     b AS x ,
                        b AS y
             ),
        d AS ( SELECT   1 AS i
               FROM     c AS x ,
                        c AS y
             ),
        e AS ( SELECT   1 AS i
               FROM     d AS x ,
                        d AS y
             ),
        f AS ( SELECT   1 AS i
               FROM     e AS x ,
                        e AS y
             ),
        numbers
          AS ( SELECT TOP ( @number_of_numbers )
                        ROW_NUMBER() OVER ( ORDER BY ( SELECT   NULL
                                                     ) ) AS number
               FROM     f
             )
    SELECT  *
    FROM    numbers;

谢谢!

Answer 1

ROW_NUMBER在ORDER BY语法上需要一个子句.没有一个你不能使用它.SELECT NULL是一个黑客在没有强制执行任何特定订单时关闭错误.在这种情况下,我们不需要强制执行任何订单,因此最快的选择是使用SELECT NULL.

优化器可以看到这个技巧,因此它没有运行时成本(通过查看执行计划可以轻松验证此声明).