将 STRING_AGG 移植到 FOR XML

Question

我目前正在处理Data Discovery and Classification，我有一个query，它允许我在进行分类时查看数据的预览。

这是一个如何与 AdventureWorks 对抗的示例：

DECLARE @TableName VARCHAR(100) = 'Product'

DROP TABLE IF EXISTS #ColumnsToDisplay

SELECT    ROW_NUMBER () OVER (ORDER BY tab.name) AS Iteration,
          SCHEMA_NAME (tab.schema_id) AS schema_name,
          tab.name AS table_name,
          --col.column_id,
          col.name AS column_name,
          --t.name AS data_type,
          --col.max_length,
          --col.precision,
          CAST(NULL AS VARCHAR(MAX)) AS DataSample
INTO      #ColumnsToDisplay
FROM      sys.tables AS tab
JOIN      sys.columns AS col
    ON    col.object_id = tab.object_id
--LEFT JOIN sys.types AS t
--    ON    col.user_type_id = t.user_type_id
WHERE     tab.name = @TableName

DECLARE @Iterations       INT = 0,
        @CurrentIteration INT = 1;

SELECT @Iterations = MAX (Iteration)
FROM   #ColumnsToDisplay 
WHILE @CurrentIteration <= @Iterations
BEGIN
    DECLARE @CurrentTableName  VARCHAR(100)   = '',
            @CurrentColumnName VARCHAR(100)   = '',
            @DynamicQuery      NVARCHAR(1000) = N''
    DECLARE @Sample VARCHAR(MAX)

    SET @CurrentTableName = '';
    SET @DynamicQuery = N'';

    SELECT @CurrentTableName = CONCAT (ttq.schema_name, '.', ttq.table_name),
           @CurrentColumnName = ttq.column_name
    FROM   #ColumnsToDisplay AS ttq
    WHERE  ttq.Iteration = @CurrentIteration

    IF (@CurrentTableName = '')
    BEGIN
        SET @CurrentIteration += 1

        CONTINUE
    END

    SET @DynamicQuery = CONCAT (N'
    SELECT @Sample = STRING_AGG(t.ColumnData,'', '') 
    FROM (
            SELECT TOP 5  CAST(x.', @CurrentColumnName, ' AS VARCHAR(MAX)) AS ColumnData 
            FROM ', @CurrentTableName, ' AS x 
            WHERE x.', @CurrentColumnName, ' IS NOT NULL
    )t')

    EXECUTE sys.sp_executesql @DynamicQuery,
                              N'@Sample VARCHAR(MAX) OUTPUT',
                              @Sample = @Sample OUTPUT

    UPDATE #ColumnsToDisplay
    SET    DataSample = @Sample
    WHERE  Iteration = @CurrentIteration

    SET @CurrentIteration += 1
END

SELECT ctd.Iteration,
       ctd.schema_name,
       ctd.table_name,
       --ctd.column_id,
       ctd.column_name,
       --ctd.data_type,
       --ctd.max_length,
       --ctd.precision,
       ctd.DataSample
FROM   #ColumnsToDisplay AS ctd

结果如下：

Iteration	schema_name	table_name	column_name	DataSample
1	Production	Product	ProductID	980, 365, 771, 404, 977
2	Production	Product	Name	Adjustable Race, All-Purpose Bike Stand, AWC Logo Cap, BB Ball Bearing, Bearing Ball
3	Production	Product	ProductNumber	AR-5381, BA-8327, BB-7421, BB-8107, BB-9108
4	Production	Product	MakeFlag	0, 0, 1, 0, 1
5	Production	Product	FinishedGoodsFlag	0, 0, 0, 0, 0
6	Production	Product	Color	Black, Black, Black, Silver, Silver

问题在于该查询仅适用于 SQL Server 2017 及更高版本，因为它使用STRING_AGG。对于 SQL Server 2016 及更低版本，我应该改用 STUFF。

我关注了这个example，但我无法真正修复它。

我唯一知道的是我需要移植的部分代码是这样的：

SET @DynamicQuery = CONCAT (N'
SELECT @Sample = STRING_AGG(t.ColumnData,'', '') 
FROM (
        SELECT TOP 5  CAST(x.', @CurrentColumnName, ' AS VARCHAR(MAX)) AS ColumnData 
        FROM ', @CurrentTableName, ' AS x 
        WHERE x.', @CurrentColumnName, ' IS NOT NULL
)t')

谁能帮我把STRING_AGG移植到STUFF？

谢谢

Answer 1

首先，让我们澄清一下 stuff 就像 Stu 提到的那样，只是用来删除第一个分隔符。 所以，如果你有

'a','b','c' 

如果你想使用逗号分隔符，xml 路径会给你：

,a,b,c

然后你用东西把第一个逗号删掉：

a,b,c

您的查询似乎根本没有分隔符，因此您不会使用stuff。考虑到这一点，这是我未经测试的猜测，在它上面添加动态查询很棘手：

SET @DynamicQuery = CONCAT (N'
    set @Sample =
            ( 
                select t.ColumnData
                from ', @CurrentTableName, ' AS x 
                where x.', @CurrentColumnName, ' IS NOT NULL
                for xml path(''''), type
            ).value(''.'',''nvarchar(max)'')
)t')

Answer 2

感谢您的帮助。

正确的问题是：

SET @DynamicQuery = CONCAT (N'
SELECT @Sample =  STUFF((SELECT '', ''+ t.ColumnData  
FROM (
        SELECT TOP 5 CAST(x.[', @CurrentColumnName, '] AS VARCHAR(MAX)) AS ColumnData 
        FROM ', @CurrentTableName, ' AS x 
        WHERE x.[', @CurrentColumnName, '] IS NOT NULL 
) AS t
   FOR XML PATH('''')),1,1,'''')')