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

Given how little of the query is being exposed; a very rough rule of thumb is to replace an Or with a Union to avoid table scanning.

Select..
LEFT JOIN Child c ON c.ParentAId = a.ParentAId 
union
Select..
left Join Child c ON c.ParentBId = b.ParentBId