<MatchKeys>
    <fuzzyKeys>
      <key key1="MatchKey_PostOut" key2="MatchKey_Name1" />
      <key key1="MatchKey_Name1" key2="MatchKey_PhoneticStreet" key3="MatchKey_PhoneticCompanyName"/>
      <key key1="MatchKey_PostOut" key2="MatchKey_PostIn" />
    </fuzzyKeys>
  </MatchKeys>

with cteMatchKeys (MatchKey_Group, MatchKey_ID, KeyColumn)
as
(
select 1, 1, 'MatchKey_PostOut' union all
select 1, 2, 'MatchKey_Name1' union all
select 2, 1, 'MatchKey_Name1' union all
select 2, 2, 'MatchKey_PhoneticStreet' union all
select 2, 3, 'MatchKey_PhoneticCompanyName' union all
select 3, 1, 'MatchKey_PostOut' union all
select 3, 2, 'MatchKey_PostIn'

)
select * from cteMatchKeys

DECLARE @x XML ='<MatchKeys>
    <fuzzyKeys>
      <key key1="MatchKey_PostOut" key2="MatchKey_Name1" />
      <key key1="MatchKey_Name1" key2="MatchKey_PhoneticStreet" key3="MatchKey_PhoneticCompanyName"/>
      <key key1="MatchKey_PostOut" key2="MatchKey_PostIn" />
    </fuzzyKeys>
  </MatchKeys>';

SELECT DENSE_RANK() OVER(ORDER BY x1.r1) AS MatchKey_Group,
       ROW_NUMBER() OVER(PARTITION BY x1.r1 ORDER BY x2.r2) AS MatchKey_ID,
       x2.r2.value('.','VARCHAR(30)') AS KeyColumn
FROM @x.nodes('/MatchKeys/fuzzyKeys/key') AS x1(r1)
CROSS APPLY x1.r1.nodes('@*') AS x2(r2);

DECLARE @xmlData XML
SET @xmlData = ('
<MatchKeys>
  <fuzzyKeys>
    <key key1="1" key2="1" key3="MatchKey_PostOut" />
  </fuzzyKeys>
  <fuzzyKeys>
    <key key1="1" key2="2" key3="MatchKey_Name1" />
  </fuzzyKeys>
  <fuzzyKeys>
    <key key1="2" key2="1" key3="MatchKey_Name1" />
  </fuzzyKeys>
  <fuzzyKeys>
    <key key1="2" key2="2" key3="MatchKey_PhoneticStreet" />
  </fuzzyKeys>
  <fuzzyKeys>
    <key key1="2" key2="3" key3="MatchKey_PhoneticCompanyName" />
  </fuzzyKeys>
  <fuzzyKeys>
    <key key1="3" key2="1" key3="MatchKey_PostOut" />
  </fuzzyKeys>
  <fuzzyKeys>
    <key key1="3" key2="2" key3="MatchKey_PostIn" />
  </fuzzyKeys>
</MatchKeys>
')

SELECT 
  x.ref.value('@key1[1]', 'int') AS MatchKey_Group,
  x.ref.value('@key2[1]', 'int') AS MatchKey_ID,
  x.ref.value('@key3[1]', 'varchar(50)') AS KeyColumn
FROM @xmlData.nodes('/MatchKeys/fuzzyKeys/key') AS x( ref )

DECLARE @xmlData XML ='<MatchKeys>
    <fuzzyKeys>
      <key key1="MatchKey_PostOut" key2="MatchKey_Name1" />
      <key key1="MatchKey_Name1" key2="MatchKey_PhoneticStreet" key3="MatchKey_PhoneticCompanyName"/>
      <key key1="MatchKey_PostOut" key2="MatchKey_PostIn" />
    </fuzzyKeys>
  </MatchKeys>';

;WITH XML_Unveiled AS (
    SELECT MatchKey_Group=ROW_NUMBER() OVER (ORDER BY (SELECT NULL)),
      x.ref.value('@key1[1]', 'varchar(50)') AS Key1,
      x.ref.value('@key2[1]', 'varchar(50)') AS Key2,
      x.ref.value('@key3[1]', 'varchar(50)') AS Key3
    FROM @xmlData.nodes('/MatchKeys/fuzzyKeys/key') AS x( ref ))
SELECT MatchKey_Group, MatchKey_ID, KeyColumn
FROM XML_Unveiled
CROSS APPLY (
    VALUES (1, Key1),(2,Key2),(3,Key3)) a (MatchKey_ID, KeyColumn)
WHERE KeyColumn IS NOT NULL 
ORDER BY MatchKey_Group, MatchKey_ID