Question

我通读了documentation中的SqlServer EXCEPT运算符，但没有看到在字符串末尾显式修剪空白的内容。但是，在运行时：

SELECT 'Test'
EXCEPT
SELECT 'Test '

没有返回结果。任何人都可以解释这种行为，或者在使用EXCEPT时如何避免这种行为？

Answer 1

ANSI SQL-92要求字符串在比较之前具有相同的长度，并且填充字符为空格。

有关更多信息，请参见https://support.microsoft.com/en-us/help/316626/inf-how-sql-server-compares-strings-with-trailing-spaces

在ANSI标准中（访问here的8.2节）

3）确定两个字符串的比较如下：

        a) If the length in characters of X is not equal to the length
          in characters of Y, then the shorter string is effectively
          replaced, for the purposes of comparison, with a copy of
          itself that has been extended to the length of the longer
          string by concatenation on the right of one or more pad char-
          acters, where the pad character is chosen based on CS. If
          CS has the NO PAD attribute, then the pad character is an
          implementation-dependent character different from any char-
          acter in the character set of X and Y that collates less
          than any string under CS. Otherwise, the pad character is a
          <space>.

        b) The result of the comparison of X and Y is given by the col-
          lating sequence CS.

        c) Depending on the collating sequence, two strings may com-
          pare as equal even if they are of different lengths or con-
          tain different sequences of characters. When the operations
          MAX, MIN, DISTINCT, references to a grouping column, and the
          UNION, EXCEPT, and INTERSECT operators refer to character
          strings, the specific value selected by these operations from
          a set of such equal values is implementation-dependent.

如果必须避免这种情况，您可以将列作为EXCEPT的一部分反转：

SELECT 'TEST', REVERSE('TEST')
EXCEPT
SELECT 'TEST ', REVERSE('TEST ')

给出了预期的结果，但是很烦人，尤其是当您处理多个列时。

另一种选择是找到带有替代填充字符或无填充选项集的整理序列，尽管快速谷歌搜索后似乎在t-sql中不存在此序列。

或者，您可以用字符终止每一列，然后最后将其子字符串化：

SELECT SUBSTRING(col,1,LEN(col) -1) FROM 
(
    SELECT 'TEST' + '^' as col
    EXCEPT 
    SELECT 'TEST ' + '^'
) results

为什么EXCEPT子句会在文本末尾修剪空白？

1 个答案: