Please explain what does "for xml path(''), TYPE) .value('.', 'NVARCHAR(MAX)')" do in this code

Question

In this code I am converting the subjects(columns) English , Hindi , Sanskrith , into rows

DECLARE @colsUnpivot AS NVARCHAR(MAX),
    @query  AS NVARCHAR(MAX)

 select @colsUnpivot 
               =stuff(
         (select ',' + quotename (C.name)
          from sys.columns c
          where c.object_id = OBJECT_ID('dbo.result2')
          for xml path(''), TYPE) .value('.', 'NVARCHAR(MAX)'),
        1, 8, ''
    );

print @colsunpivot

select  @query 
              = 'select Name, Subject,Marks
                 from result2
                 unpivot
                        (
                          marks for subject in (' + @colsunpivot + ')) as tab'

exec sp_executesql @query;

Please explain what does for xml path(''), TYPE) .value('.', 'NVARCHAR(MAX)') do here .

Answer 1

A common technique for aggregate string concatenation before SQL Server 2017 is using XML data type methods along with STUFF to remove the extra delimiter. In SQL Server 2017 and later, STRING_AGG makes this task much cleaner and easier. The STRING_AGG equivalent:

select @colsUnpivot = STRING_AGG(quotename (C.name),',')
          from sys.columns c
          where c.object_id = OBJECT_ID('dbo.result2')

The components of the XML expression technique for aggregate string concatenation are as follows.

This subquery will generate a result set with one row per column in the source table with a comma preceding each column name:

select ',' + quotename (C.name)
          from sys.columns c
          where c.object_id = OBJECT_ID('dbo.result2')

Example result:

,[column1]
,[column2]
,[column3]

Adding the FOR XML PATH(''), TYPE converts these rows into a single strongly-typed XML text node with the concatenated rows, allowing XML methods like value to be used on the type:

select ',' + quotename (C.name)
          from sys.columns c
          where c.object_id = OBJECT_ID('dbo.result2')
for xml path(''), TYPE

Result (XML type):

,[column1],[column2],[column3]

Invoking the method value('.', 'NVARCHAR(MAX)') on that XML node converts the XML node to an nvarchar(MAX) string. The value XML data type method takes an XQuery expression with '.' representing the current node (only node here) and the second argument being the desired SQL data type to be returned.

SELECT
         (select ',' + quotename (C.name)
          from sys.columns c
          where c.object_id = OBJECT_ID('dbo.result2')
          for xml path(''), TYPE).value('.', 'NVARCHAR(MAX)')

Result (nvarchar(MAX)):

,[column1],[column2],[column3]

Finally, the STUFF function removes the extraneous leading delimiter from the string. There is an error in your original example; the length should be 1 instead of 8 so that only the comma is removed:

SELECT
         STUFF((select ',' + quotename (C.name)
          from sys.columns c
          where c.object_id = OBJECT_ID('dbo.result2')
          for xml path(''), TYPE).value('.', 'NVARCHAR(MAX)'),1,1,'')

Final result (nvarchar(MAX)):

[column1],[column2],[column3]

I sometimes see folks omit the , TYPE).value('.', 'NVARCHAR(MAX)') when using this technique. The problem with that is that some characters must be escaped with XML entity references (e.g; quotes) so the resultant string will not be as expected in that case.

EDIT:

结合 Mister Magoo 的建议使用(./text())[1]而不是仅仅.作为节点规范，下面的查询将提高大型数据集的性能。

SELECT
         STUFF((select ',' + quotename (C.name)
          from sys.columns c
          where c.object_id = OBJECT_ID('dbo.result2')
          for xml path(''), TYPE).value('(./text())[1]', 'NVARCHAR(MAX)'),1,1,'');

Answer 2

正如丹所说，但也...

养成在 xml 值中使用 text() 节点的习惯，它不会对小结果集产生太大影响，但我认为没有理由不使用它，并且您将真正看到更大数据集的好处.

红色部分是痛苦的，可以通过使用 (./text())[1] 来避免