FOR XML PATH('')在连接行时的工作原理

Question

FOR XML PATH ('')在SQL Server中连接行时,该子句如何工作？

我只想解释该FOR XML PATH ('')条款的工作原理......

Answer 1

是什么FOR XML PATH('xxx')做的是创建为使每行中的结果集的XML字符串<xxx></xxx>元素和行内的每个列的值,在元素的名称该列.

如果PATH为空(即PATH(''))它省略了XML生成中的row元素.如果列没有名称,则省略XML生成中的列元素.当两个PATH都为空且列没有名称时,它实际上变为所有行的字符串连接.

运行以下语句以更好地了解该过程:

-- Each row is in a <beta></beta> element
-- Each column in that row in a <alfa></alfa> element (the column name)
SELECT
    alfa=','+TABLE_SCHEMA + '.' + TABLE_NAME
FROM
    INFORMATION_SCHEMA.TABLES
FOR
    XML PATH('beta');

-- Since the PATH is empty, the rows are not put inside an element
-- Each column in that row is in a <alfa></alfa> element (the column name)
SELECT
    alfa=','+TABLE_SCHEMA + '.' + TABLE_NAME
FROM
    INFORMATION_SCHEMA.TABLES
FOR
    XML PATH('');

-- Since the PATH is empty, the rows are not put inside an element
-- Since the column has no name it is not put inside an element     
SELECT
    ','+TABLE_SCHEMA + '.' + TABLE_NAME
FROM
    INFORMATION_SCHEMA.TABLES
FOR
    XML PATH('');

-- This uses the STUFF function to remove the leading comma to get a proper comma-seperated list    
SELECT STUFF((
    SELECT
        ','+TABLE_SCHEMA + '.' + TABLE_NAME
    FROM
        INFORMATION_SCHEMA.TABLES
    FOR
        XML PATH('')
    ),1,1,''
) AS comma_seperated_list;

现在我听到你问:当我只是从表中选择一列时,如何删除列名.根据我的偏好,有几种方法:

• XQuery属性: SELECT [text()]=column_name ...
• 使用子查询选择列值: SELECT (SELECT column_name) ...
• 将列加入其类型: SELECT CAST(column_value AS <TYPE of the column>) ...

例子:

SELECT
    [text()]=TABLE_NAME
FROM
    INFORMATION_SCHEMA.TABLES
FOR
    XML PATH('');

SELECT
    (SELECT TABLE_NAME)
FROM
    INFORMATION_SCHEMA.TABLES
FOR
    XML PATH('');

SELECT
    CAST(TABLE_NAME AS SYSNAME)
FROM
    INFORMATION_SCHEMA.TABLES
FOR
    XML PATH('');