SQL Server 2019

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DECLARE @xml XML = 
N'<ENVELOPE>
    <BILLFIXED>
        <BILLDATE>29-Jun-2019</BILLDATE>
        <BILLREF>123</BILLREF>
        <BILLPARTY>ABC</BILLPARTY>
    </BILLFIXED>
    <BILLOP>200</BILLOP>
    <BILLCL></BILLCL>
    <BILLDUE></BILLDUE>
    <BILLFIXED>
        <BILLDATE>30-Jun-2019</BILLDATE>
        <BILLREF>April To June -19</BILLREF>
        <BILLPARTY>efg</BILLPARTY>
    </BILLFIXED>
    <BILLOP>100</BILLOP>
    <BILLCL>100</BILLCL>
    <BILLDUE>30-Jun-2019</BILLDUE>
    <BILLOVERDUE>1115</BILLOVERDUE>
</ENVELOPE>';

SELECT
      c.value('(BILLDATE /text())[1]', 'VARCHAR(20)') AS BILLDATE
    , c.value('(BILLREF  /text())[1]', 'VARCHAR(20)') AS BILLREF
    , c.value('(BILLPARTY/text())[1]', 'VARCHAR(20)') AS BILLPARTY
    , c.value('let $i := . let $nxt := (/ENVELOPE/BILLFIXED[. >> $i])[1] return (/ENVELOPE/BILLOP     [. >> $i][. << $nxt]/text())[1]', 'INT') AS BILLOP
    , c.value('let $i := . let $nxt := (/ENVELOPE/BILLFIXED[. >> $i])[1] return (/ENVELOPE/BILLCL     [. >> $i][. << $nxt]/text())[1]', 'INT') AS BILLCL
    , c.value('let $i := . let $nxt := (/ENVELOPE/BILLFIXED[. >> $i])[1] return (/ENVELOPE/BILLDUE    [. >> $i][. << $nxt]/text())[1]', 'VARCHAR(20)') AS BILLDUE
    , c.value('let $i := . let $nxt := (/ENVELOPE/BILLFIXED[. >> $i])[1] return (/ENVELOPE/BILLOVERDUE[. >> $i][. << $nxt]/text())[1]', 'INT') AS BILLOVERDUE
FROM @xml.nodes('/ENVELOPE/BILLFIXED') AS t(c);

BILLDATE	BILLREF	BILLPARTY	BILLOP	BILLCL	BILLDUE	BILLOVERDUE
29-Jun-2019	123	ABC	200	null	null	null
30-Jun-2019	April To June -19	efg	null	null	null	null