sql-server - “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。部分是什么order by (select null)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在语法上需要一个ORDER BY子句。没有一个你不能使用它。SELECT NULL是一种在不执行任何特定命令的情况下关闭错误的技巧。在这种情况下，我们不需要强制执行任何命令，因此最快的选择是使用SELECT NULL.

优化器看穿了这个技巧，所以它没有运行时成本（这个说法很容易通过查看执行计划来验证）。