sql - 解析xml时如何使用 "cross apply"？

Question

尝试解析多个 xml 文件。 但在应用交叉应用时堆栈

第一个变体工作正常

DECLARE @input XML = 

'<a>
    <b>
     <g>one</g>
    <n>
        <c>somedata1 1</c>
    </n>
    <n>
        <c>2somedata 2</c>
    </n>
</b>
</a>'       

SELECT
g                   = XC.value('(g)[1]', 'varchar(100)'),
i                   = xc1.n1.value ('(c)[1]', 'varchar(100)')
FROM @input.nodes ('a/b') as XT (XC)
cross apply @input.nodes ('//n') as xc1(n1)

第二个变体失败(返回空字段) - 我知道这是因为缺少 <с> 节点

DECLARE @input XML = 
'<a>
    <b>
     <g>one</g>
        </b>
</a>'
SELECT
    g                   = XC.value('(g)[1]', 'varchar(100)'),
    i                   = xc1.n1.value ('(c)[1]', 'varchar(100)')
FROM @input.nodes ('a/b') as XT (XC)
cross apply @input.nodes ('//n') as xc1(n1)

我处理多个文件，其中一些有这些节点，有些没有，我的决定应该是什么？

Answer 1

在第二种情况下，您没有任何 n 元素。您需要使用OUTER APPLY :

There are two forms of APPLY: CROSS APPLY and OUTER APPLY. CROSS APPLY returns only rows from the outer table that produce a result set from the table-valued function. OUTER APPLY returns both rows that produce a result set, and rows that do not, with NULL values in the columns produced by the table-valued function.

DECLARE @input XML = 
'<a>
    <b>
     <g>one</g>
        </b>
</a>';

SELECT g = XC.value('(g)[1]', 'varchar(100)'),
       i = xc1.n1.value ('(c)[1]', 'varchar(100)')
FROM @input.nodes ('a/b') AS XT (XC)
OUTER APPLY @input.nodes ('//n') AS xc1(n1);

LiveDemo

输出:

╔═════╦══════╗
║  g  ║  i   ║
╠═════╬══════╣
║ one ║ NULL ║
╚═════╩══════╝