如果查询包含记录，则根据查询创建 xml

Question

我正在尝试根据 Oracle SQL 中的查询创建一些 xml。如果您进行 1 对 1 转换，这将非常容易，但是我需要围绕 returned xml 创建一个分组元素。这是我的工作代码。

WITH table2 AS (
  SELECT t.id
  FROM   table1 t
  WHERE t.code = 'somevalue')

select xmlelement("mes:tests",
    xmlattributes('http://testnamespace.com/' as "xmlns:tns"),
    xmlagg(xmlelement("tns:test",
        xmlelement("tns:ID", id))))
from table2;

问题是在这种情况下它 return 是一个空的测试元素，即使没有数据。我需要它 return NULL。到目前为止，我已经尝试了几件事。例如这种情况：

WITH table2 AS (
      SELECT t.id
      FROM   table1 t
      WHERE t.code = 'somevalue')

select CASE WHEN exists(select id from table2) THEN 
 xmlelement("mes:tests",
        xmlattributes('http://testnamespace.com/' as "xmlns:tns"),
        xmlagg(xmlelement("tns:test",
            xmlelement("tns:ID", id)))) ELSE null end
    from table2;

然而这给出了一个错误：

not a single-group group function

有谁知道我可以这样做的好方法吗？我正在使用 Oracle DB 12c。

Answer 1

根据您的描述，您可以将 exists 检查更改为 count:

select CASE WHEN count(id) > 0 THEN 
 xmlelement("mes:tests",
        xmlattributes('http://testnamespace.com/' as "xmlns:tns"),
        xmlagg(xmlelement("tns:test",
            xmlelement("tns:ID", id)))) ELSE null end
    from table2;

使用另一个 CTE 进行示例数据演示，没有匹配项：

set null "(null)"

WITH table1(id, code) as (
      select 42, 'ignore' from dual),
table2 AS (
      SELECT t.id
      FROM   table1 t
      WHERE t.code = 'somevalue')

select CASE WHEN count(id) > 0 THEN 
 xmlelement("mes:tests",
        xmlattributes('http://testnamespace.com/' as "xmlns:tns"),
        xmlagg(xmlelement("tns:test",
            xmlelement("tns:ID", id)))) ELSE null end
    from table2;

CASEWHENCOUNT(ID)>0THENXMLELEMENT("MES:TESTS",XMLATTRIBUTES('HTTP://TESTNAMESPAC
--------------------------------------------------------------------------------
(null)

并匹配：

WITH table1(id, code) as (
      select 42, 'somevalue' from dual union all
      select 43, 'somevalue' from dual),
table2 AS (
      SELECT t.id
      FROM   table1 t
      WHERE t.code = 'somevalue')

select CASE WHEN count(id) > 0 THEN 
 xmlelement("mes:tests",
        xmlattributes('http://testnamespace.com/' as "xmlns:tns"),
        xmlagg(xmlelement("tns:test",
            xmlelement("tns:ID", id)))) ELSE null end
    from table2;

CASEWHENCOUNT(ID)>0THENXMLELEMENT("MES:TESTS",XMLATTRIBUTES('HTTP://TESTNAMESPAC
--------------------------------------------------------------------------------
<mes:tests xmlns:tns="http://testnamespace.com/"><tns:test><tns:ID>42</tns:ID></
tns:test><tns:test><tns:ID>43</tns:ID></tns:test></mes:tests>

如果查询包含记录，则根据查询创建 xml

Create xml from query if query contains records

case

oracle12c

sqlxml