Oracle 21c

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-- not singly-rooted XML
SELECT 
  XMLSERIALIZE(
   DOCUMENT xmltype('<bar><val x="y">42</val></bar><foo></foo>') AS VARCHAR(100) 
   INDENT
  )
FROM dual;
ORA-31011: XML parsing failed
ORA-19213: error occurred in XML processing at lines 1
LPX-00245: extra data after end of document
ORA-06512: at "SYS.XMLTYPE", line 310
ORA-06512: at line 1

-- not singly-rooted XML
SELECT 
  XMLSERIALIZE(
   CONTENT xmltype('<bar><val x="y">42</val></bar><foo></foo>') AS VARCHAR(100) 
   INDENT
  )
FROM dual; 
ORA-31011: XML parsing failed
ORA-19213: error occurred in XML processing at lines 1
LPX-00245: extra data after end of document
ORA-06512: at "SYS.XMLTYPE", line 310
ORA-06512: at line 1

-- singly-rooted XML (DOCUMENT)
SELECT 
  XMLSERIALIZE(
   DOCUMENT xmltype('<?xml version="1.0" encoding="ISO-8859-1"?><foo><bar><val>&#52;&#50;</val></bar></foo>') AS VARCHAR(100) 
   INDENT
  ) AS xml
FROM dual; 


XML


<?xml version="1.0" encoding="UTF-8"?>
<foo>
  <bar>
    <val>42</val>
  </bar>
</foo>



-- singly-rooted XML (CONTENT)
SELECT 
  XMLSERIALIZE(
   CONTENT xmltype('<?xml version="1.0" encoding="ISO-8859-1"?><foo><bar><val>&#52;&#50;</val></bar></foo>') AS VARCHAR(100)
   INDENT
  ) AS xml
FROM dual; 


XML


<?xml version="1.0" encoding="UTF-8"?>
<foo>
  <bar>
    <val>42</val>
  </bar>
</foo>



-- singly-rooted XML (DOCUMENT) without declaration
SELECT 
  XMLSERIALIZE(
   DOCUMENT xmltype('<foo><bar><val>&#52;&#50;</val></bar></foo>') AS VARCHAR(100) 
   INDENT
  ) AS xml
FROM dual; 


XML


<foo>
  <bar>
    <val>42</val>
  </bar>
</foo>



-- singly-rooted XML (CONTENT) without declaration
SELECT 
  XMLSERIALIZE(
   CONTENT xmltype('<foo><bar><val>&#52;&#50;</val></bar></foo>') AS VARCHAR(100) 
   INDENT
  ) AS xml
FROM dual; 


XML


<foo>
  <bar>
    <val>42</val>
  </bar>
</foo>