merging two documents - only if second document matche

I have two documents, file A and file B.  I want to join them on the id of
the first, but only if a matching id is in the 2nd.  How do I do this?

File A              File B               Desired Output
<id> A </id>        <id> A </id>         <id> A </id>
<id> B </id>        <id> C </id>         <id> D </id>
<id> D </id>        <id> D </id>

Thanks,
     Matt Youngblut


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