Select all parents or children in same table relation SQL Server

Question

SQL developers, I have a badly planned database as task to learn a lot about SQL Server 2012.

SO, there is the table Elem:

+-----------+----         


        

Answer 1

I dont think it can be done in one select in whole case ,so that you can select all parents,grandparents ,... . One way how to do it is to join elem table to herself ,and thet it depends on how many levels of join you do , that level of childs,grandchilds will you get.

The solution can be somethink like this(for the second case )

this will select you all parents,childs and grandchilds

Select 
parent.key as parent_key,
child.key as child_key,
grandchild.key as grandchild_key 
from elem parent 
join elem child on (elem.key=child.parentkey)
join elem grandchild on (child.key=grandchild.parentkey)
where parent.parentkey is null; -- this make you sure that first level will be parents

solution for the first case is just that you will connect tables not in style of 'key=parentkey' but oposite 'parentkey=key'.