sub node in a table in sql server 2008

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I have structure like this:


<Unit>
  <SubUnit1>
           <SubSubUnit1/>
           <SubSubUnit2/>
           ...
           <SubSubUnitN/>
  </SubUnit1/>
  <SubUnit2>
           <SubSubUnit1/>
           <SubSubUnit2/>
           ...
           <SubSubUnitN/>
  </SubUnit2/>
  ...
  <SubUnitN>
           <SubSubUnit1/>
           <SubSubUnit2/>
           ...
           <SubSubUnitN/>
  </SubUnitN/>
</Unit>
This structure has 3 levels: main Unit, SubUnits and SubSubUnits.

I want to select all children by UnitId.
If I search by Unit, I have to get all tree.
If I search by SubUnit1, I have to get SubUnit1 and all children of SubUnit1.
If I search SubSubUnit2, I have to get itself.