Oracle 11g上XMLTABLE的奇怪错误

Question

我正在使用Oracle 11.2.0.4.0并且在涉及XMLTABLE时已经运行了几次问题。 我的最新问题可以通过以下示例（我设计为尽可能简单）来演示：

with data as
(
  select '<A><B>B21</B></A>' x from dual
),
extractedxml as (
  SELECT b
  FROM data d,
       xmltable('/A/B' PASSING xmltype(d.x) COLUMNS b varchar2(20) PATH '.')
)
select b from extractedxml union 
select b from extractedxml;

产生以下错误：

ORA-19032: Expected XML tag , got no content
ORA-06512: at "SYS.XMLTYPE", line 310
ORA-06512: at line 1
19032. 00000 -  "Expected XML tag %s got %s"
*Cause:    When converting XML to object, a wrong tag name was present
*Action:   Pass a valid canonical XML that can map to the given object type

而以下查询按预期工作（with子句未更改）：

with data as
(
  select '<A><B>B21</B></A>' x from dual
),
extractedxml as (
  SELECT b
  FROM data d,
       xmltable('/A/B' PASSING xmltype(d.x) COLUMNS b varchar2(20) PATH '.')
)
select b from extractedxml;

B
--------------------
B21

如果避免使用XMLTABLE ，查询仍然有效：

with data as
(
  select '<A><B>B21</B></A>' x from dual
),
extractedxml as (
  SELECT cast (extractvalue(column_value,'B') as varchar2(20)) b
  FROM data, table(xmlsequence(extract(xmltype(data.x),'/A/B')))
)
select b from extractedxml union 
select b from extractedxml;

B
--------------------
B21

所以我有一个解决方法，只要我不理解上述行为，我就会避免使用XMLTABLE 。 XMLTABLE被认为是错误的还是我错过了什么？

Answer 1

根据我的经验，最好为xmltable添加另一个标准列。

这个SQL语句工作正常：

with data as
(
  select '<A><B>B21</B></A>' x from dual
),
extractedxml as (
  SELECT b
  FROM data d,
       xmltable('/A/B' PASSING xmltype(d.x) COLUMNS i FOR ORDINALITY, b varchar2(20) PATH '.')
)
select b from extractedxml union 
select b from extractedxml;

省略通用栏引起的另一个致命问题：

with data as
(
  select xmltype('<A><B>B21</B></A>') x from dual
),
extractedxml as (
  SELECT b
  FROM data d,
       xmltable('/A/B' PASSING d.x COLUMNS b varchar2(20) PATH '.')
)
select b from extractedxml union 
select b from extractedxml;

>> no result (!)

但

with data as
(
  select xmltype('<A><B>B21</B></A>') x from dual
),
extractedxml as (
  SELECT b
  FROM data d,
       xmltable('/A/B' PASSING d.x COLUMNS i FOR ORDINALITY, b varchar2(20) PATH '.')
)
select b from extractedxml union 
select b from extractedxml;

>> B21