即使表达式值为空，如何执行xmlforest creat元素 [英] how to enforce xmlforest creat elements even if expression value is null

问题描述

我想使用oracle XMLFOREST函数. (定义链接)

I would like to use the oracle XMLFOREST function. (Link for definition)

语法示例:

XMLFOREST( [value expression AS alias], [...])

我的问题是，如果值表达式为NULL，则不会为该值表达式创建任何元素.

My problem is that if value expression is NULL, then no element is created for that value expression.

如何强制它添加一个空的XML片段?

How can I enforce it to add an empty XML fragment?

推荐答案

您可以使用

You can' use XMLFOREST:

如果value_expr为null，则不会为该value_expr创建任何元素.

If value_expr is null, then no element is created for that value_expr.

也没有类似的 XMLCOLATTVAL 功能:

Nor the similar XMLCOLATTVAL function:

您必须为value_expr指定一个值.如果value_expr为null，则不返回任何元素.

You must specify a value for value_expr. If value_expr is null, then no element is returned.

另一方面，

XMLELEMENT 手将按要求返回一个空元素:

XMLELEMENT on the other hand returns an empty element as requested:

SQL> select xmlelement("EMP", xmlelement("ENAME", ename), 
  2                           xmlelement("JOB", job),
  3                           xmlelement("MGR", mgr)
  4         ).getclobval() xml
  5    from scott.emp;

XML
----------------------------------------------------------------
<EMP><ENAME>KING</ENAME><JOB>PRESIDENT</JOB><MGR></MGR></EMP>
<EMP><ENAME>BLAKE</ENAME><JOB>MANAGER</JOB><MGR>7839</MGR></EMP>
...

---

为了完整起见，还可以使用另外两个Oracle工具(灵感来自这篇文章在OTN论坛上).首先，您可以使用 XMLQUERY (在11.2上吗?):

---

For completeness, two more Oracle tools can also be used (inspired by this post on the OTN forums). First you can use an XMLQUERY (on 11.2? ):

SQL> select
  2    xmlquery(
  3     '(#ora:view_on_null empty #) {
  4      <EMPS> {
  5          for $c in fn:collection("oradb:/SCOTT/EMP")/ROW
  6          return element EMP {
  7            $c/ENAME
  8          , $c/JOB
  9          , $c/MGR
 10       }
 11      }</EMPS>
 12     }'
 13    passing cast(10 as number) as "mid"
 14    returning content
 15  ).getClobval() as result
 16  from dual;

RESULT 
----------------------------------------------------------------------
<EMPS>
   <EMP><ENAME>KING</ENAME><JOB>PRESIDENT</JOB><MGR></MGR></EMP>
   <EMP><ENAME>BLAKE</ENAME>...

您还可以使用 DBMS_XMLGEN 包:

You can also use the DBMS_XMLGEN package:

SQL> DECLARE
  2    ctx    dbms_xmlgen.ctxHandle;
  3    sqlstr varchar2(4000)
  4       := 'SELECT ename, job, mgr FROM scott.emp WHERE ename=''KING''';
  5    res    clob;
  6  BEGIN
  7    ctx := dbms_xmlgen.newContext(sqlstr);
  8    dbms_xmlgen.setNullHandling(ctx, dbms_xmlgen.EMPTY_TAG);
  9    res := dbms_xmlgen.getXML(ctx);
 10    dbms_xmlgen.closeContext(ctx);
 11    dbms_output.put_line(res);
 12  END;
 13  /

<?xml version="1.0"?>
<ROWSET>
 <ROW>
  <ENAME>KING</ENAME>
  <JOB>PRESIDENT</JOB>
  <MGR/>
 </ROW>
</ROWSET>

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