Question

我想在cte中提供ROW_NUMBER能够从具有给定索引的序列中选择特定项目（如数组索引器arr[index]）。当然，“序列”是一个来自表值函数（Split）的表。

那么如何在索引不可用时创建索引呢？这是查询：

DECLARE @string VARCHAR(100); 
SET @string='field1;field2;field3;field4;field5;field6;field7'; 
DECLARE @index INT; 
SET @index = 4; 

WITH cte 
     AS (SELECT item, 
                rn=Row_number() 
                     OVER( 
                       ORDER BY item) 
         FROM   dbo.Split(@string, ';')) 
SELECT TOP 1 item 
FROM   cte 
WHERE  rn = @index

Row_number() OVER(ORDER BY item)按字母顺序排序，但实际上我想按（不可用）索引排序。

请注意，此问题出现在my answer的this so-question上。

Answer 1

当然，您只需要跟踪列表中每个项目的显示位置。一种方法是让您的split函数插入IDENTITY列。考虑my answer from the question you reference：

CREATE FUNCTION dbo.SplitStrings
(
    @List       NVARCHAR(MAX),
    @Delimiter  NVARCHAR(255)
)
RETURNS @t TABLE([Index] INT IDENTITY(1,1), Item NVARCHAR(255))
AS
BEGIN
    INSERT @t(Item) SELECT SUBSTRING(@List, Number, 
        CHARINDEX(@Delimiter, @List + @Delimiter, Number) - Number)
    FROM (SELECT ROW_NUMBER() OVER (ORDER BY [object_id])
        FROM sys.all_objects) AS n(Number)
    WHERE Number <= CONVERT(INT, LEN(@List))
        AND SUBSTRING(@Delimiter + @List, Number, 1) = @Delimiter
    ORDER BY Number OPTION (MAXDOP 1);

    RETURN;
END
GO

DECLARE @x TABLE(i INT, string NVARCHAR(4000));

INSERT @x SELECT 1, N'field1;field2;field3;field4;field5;'
UNION ALL SELECT 2, N'x;y;6;r;3;2;w;'
UNION ALL SELECT 3, N'ttt;444;rrr;333;111;444;777;888;';

SELECT x.i, s1.Item
FROM @x AS x
CROSS APPLY dbo.SplitStrings(x.string, ';') AS s1
WHERE s1.[Index] = 4;

内联替代方案，可能 稍微大规模友好：

CREATE FUNCTION dbo.SplitStrings ( @List NVARCHAR(MAX), @Delimiter NVARCHAR(255) ) RETURNS TABLE AS RETURN (SELECT [Index] = ROW_NUMBER() OVER (ORDER BY Number), Item FROM (SELECT Number, Item = LTRIM(RTRIM(SUBSTRING(@List, Number, CHARINDEX(@Delimiter, @List + @Delimiter, Number) - Number))) FROM (SELECT ROW_NUMBER() OVER (ORDER BY [object_id]) FROM sys.all_objects) AS n(Number) WHERE Number <= CONVERT(INT, LEN(@List)) AND SUBSTRING(@Delimiter + @List, Number, LEN(@Delimiter)) = @Delimiter ) AS y); GO

当然，最终您要修复架构。如果此列表中的第4个元素是重要数据，则应单独存储。

Answer 2

如果字符串中的值是唯一的，则可以在ORDER BY charindex(';'+item+';', ';'+@string+';')子句中使用OVER()。

DECLARE @string VARCHAR(100); 
SET @string='field1;field2;field3;field4;field5;field6;field7'; 
DECLARE @index INT; 
SET @index = 4; 

WITH cte 
     AS (SELECT item, 
                rn=Row_number() 
                     OVER( 
                       ORDER BY charindex(';'+item+';', ';'+@string+';')) 
         FROM   dbo.Split(@string, ';')) 
SELECT TOP 1 item 
FROM   cte 
WHERE  rn = @index

charindex会针对每个已提取的@string返回item中的位置，因此根据rn中的位置生成@string。

SQL Fiddle

拆分列表时识别第n项？

2 个答案: