How to parse xml containing optional elements

SEPA ISO XML transactions file needs to be parsed into flat table in
Postgres 9.1+ in ASP:NET 4.6 MVC controller.

I tried code below but this produces wrong result:

tasusumma orderinr
150.00 PV04131
0.38 PV04131

Since there is no EndToEnd in second row there should be null in second row
orderinr column. Correct result is:

tasusumma orderinr
150.00 PV04131
0.38 null

How to fix this ?

create temp table t(x xml, nsa text[][]) on commit drop;
insert into t values(
'<?xml version=''1.0'' encoding=''UTF-8''?>
<Document xmlns="urn:iso:std:iso:20022:tech:xsd:camt.053.001.02">
<BkToCstmrStmt>
<Stmt>
<Ntry>
<Amt Ccy="EUR">150.00</Amt>
<NtryDtls>
<TxDtls>
<Refs>
<EndToEndId>PV04131</EndToEndId>
</Refs>
</TxDtls>
</NtryDtls>
</Ntry>
<Ntry>
<Amt Ccy="EUR">0.38</Amt>
<NtryDtls>
<TxDtls>
<Refs>
<AcctSvcrRef>2016080100178214-2</AcctSvcrRef>
</Refs>
</TxDtls>
</NtryDtls>
</Ntry>
</Stmt>
</BkToCstmrStmt>
</Document>
', ARRAY[ARRAY['ns','urn:iso:std:iso:20022:tech:xsd:camt.053.001.02']]);

SELECT
unnest(xpath('/ns:Document/ns:BkToCstmrStmt/ns:Stmt/ns:Ntry/ns:Amt/text()',
x,nsa))::text::numeric AS tasusumma
,
unnest(xpath('/ns:Document/ns:BkToCstmrStmt/ns:Stmt/ns:Ntry/ns:NtryDtls/ns:TxDtls/ns:Refs/ns:EndToEndId/text()',
x,nsa))::text AS orderinr
FROM t;

Parsing can done in other ways e.q. using xslt stylesheet for tranformation
or in client side ASP.NET 4.6 MVC if this is more reasonable.

Posted also in
http://stackoverflow.com/questions/38888739/how-to-parse-xml-with-optional-elements

Andrus.