sql - 如何在T-SQL中透视XML列的属性

我需要对表中的XML列执行数据透视，其中XML包含具有多个属性的多个元素。每个元素中的属性始终相同，但是元素的数量会有所不同。让我举个例子...

FormEntryId |               FormXML                                    | DateCreated
====================================================================================
1           |<Root>                                                    | 10/15/2009
            |  <Form>                                                  |
            |    <FormData FieldName="Username" FieldValue="stevem" /> |
            |    <FormData FieldName="FirstName" FieldValue="Steve" /> |
            |    <FormData FieldName="LastName" FieldValue="Mesa" />   |
            |  </Form>                                                 |
            |</Root>                                                   |
            |                                                          |
------------------------------------------------------------------------------------
2           |<Root>                                                    | 10/16/2009
            |  <Form>                                                  |
            |    <FormData FieldName="Username" FieldValue="bobs" />   |
            |    <FormData FieldName="FirstName" FieldValue="Bob" />   |
            |    <FormData FieldName="LastName" FieldValue="Suggs" />  |
            |    <FormData FieldName="NewField" FieldValue="test" />   |
            |  </Form>                                                 |
            |</Root>                                                   |

我需要为每个不同的FieldName属性值(在此示例中为Username，FirstName，LastName和NewField)使用相应的FieldValue属性作为值的结果集结束。我上面给出的示例的结果如下所示:

FormEntryId | Username | FirstName | LastName | NewField | DateCreated
======================================================================
1           | stevem   | Steve     | Mesa     | NULL     | 10/15/2009
----------------------------------------------------------------------
2           | bobs     | Bob       | Suggs    | test     | 10/16/2009

我想出了一种使用静态列来完成此操作的方法

SELECT
    FormEntryId,
    FormXML.value('/Root[1]/Form[1]/FormData[@FieldName="Username"][1]/@FieldValue','varchar(max)') AS Username,
    FormXML.value('/Root[1]/Form[1]/FormData[@FieldName="FirstName"][1]/@FieldValue','varchar(max)') AS FirstName,
    FormXML.value('/Root[1]/Form[1]/FormData[@FieldName="LastName"][1]/@FieldValue','varchar(max)') AS LastName,
    FormXML.value('/Root[1]/Form[1]/FormData[@FieldName="NewField"][1]/@FieldValue','varchar(max)') AS NewField,
    DateCreated
FROM FormEntry

但是，我想看看是否有一种方法可以基于“FieldName”属性值的不同集合来使列动态化。

最佳答案

看一看this dynamic pivot以及最近的this one-您基本上需要能够使用SELECT DISTINCT FieldName才能使用此技术动态地构建查询。

这是您特定问题的完整答案(请注意，从不同属性生成列表时，如果知道列应显示的顺序，则存在列顺序弱点):

DECLARE @template AS varchar(MAX)
SET @template = 'SELECT
    FormEntryId
    ,{@col_list}
    ,DateCreated
FROM FormEntry'

DECLARE @col_template AS varchar(MAX)
SET @col_template = 'FormXML.value(''/Root[1]/Form[1]/FormData[@FieldName="{FieldName}"][1]/@FieldValue'',''varchar(max)'') AS {FieldName}'

DECLARE @col_list AS varchar(MAX)

;WITH FieldNames AS (
    SELECT DISTINCT FieldName
    FROM FormEntry
    CROSS APPLY (
        SELECT X.FieldName.value('@FieldName', 'varchar(255)')
        FROM FormXML.nodes('/Root[1]/Form[1]/FormData') AS X(FieldName)
    ) AS Y (FieldName)
)
SELECT @col_list = COALESCE(@col_list + ',', '') + REPLACE(@col_template, '{FieldName}', FieldName)
FROM FieldNames

DECLARE @sql AS varchar(MAX)
SET @sql = REPLACE(@template, '{@col_list}', @col_list)

EXEC (@sql)