SQL Server Left Join With 'Or' Operator

Question

I have a four tables, TopLevelParent, two mid level tables MidParentA and MidParentB, and a Child table which can have a parent of MidParentA or MidParentB (One or the other

Answer 1

another way to write it:

LEFT JOIN Child c ON c.ParentAId = COALESCE(a.ParentAId, b.ParentBId)

Edit

One possible approach is querying first the MidParentA and then the MidParentB and then UNION the results:

SELECT tlp.*,
       a.MidParentAId,
       null MidParentBId,
       c.ChildId
FROM TopLevelParent tlp
LEFT JOIN MidParentA a ON tlp.TopLevelPatientId = a.TopLevelPatientId
LEFT JOIN Child c ON c.MidParentAId = a.MidParentAId 
UNION
SELECT tlp.*,
       null MidParentAId,
       b.MidParentBId,
       c.ChildId
FROM TopLevelParent tlp
LEFT JOIN MidParentB b ON tlp.TopLevelPatientId = b.TopLevelPatientId
LEFT JOIN Child c ON c.MidParentBId = b.MidParentBId 

A demo in SQLFiddle