Question

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

我只想解释FOR XML PATH ('')子句如何工作......

Answer 1

FOR XML PATH('xxx')所做的是为结果集创建一个XML字符串，该字符串将每一行放在<xxx></xxx>元素中，每个列值放在该行中，该元素具有该列的名称。

如果PATH为空（即PATH('')），则省略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，它实际上变成了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) ...
将列标注为其类型：SELECT CAST(column_value AS <TYPE of the column>) ...

示例：

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('');

FOR XML PATH（＆＃39;＆＃39;）在连接行时的工作原理

1 个答案: