Question

I have an adjacent list hierarchy model that makes up a topic structure

   ID   Parent_Id Topic_Name
   1    Null      Topic 1
   2    Null      Topic 2
   3    2            Topic 3
   4    3               Topic 4
   5    2            Topic 5
   6    Null      Topic 6

This forms part of an application which I cant change – the topics dont have multiple parents so unfortunatly I can’t move to a nested sets – although if this was an interim step in the process – this would be fine as long as it went back to adjacent list hierarchy model

I want to specify a topic id and then copy it to a new topic id and retain the levels / structure underneath

So in my example I could specify topic topic_id 2 and it would create

   ID   Parent_Id Topic_Name
   7    Null      Topic 2
   8    7            Topic 3
   9    8               Topic 4
   10   7            Topic 5

Auto numbering is taken care of for the ID so no need to construct that, but obviously the parent id needs to be retained

How can I achieve the above? would I need to flatten the data and do 3 seperate inserts logging the id after each insert?

Answer 1

You can use a recursive CTE to get the rows to insert.
If you use merge to add the rows you can use output to capture a mapping between the generated ID and the old ID which can be used to update the column Parent_ID for the inserted rows.

-- ID for topic to copy 
declare @ID int;
set @ID = 2;

-- Table to hold the inserted rows
declare @T table
(
  New_ID int,
  Old_ID int,
  Old_ParentID int
);

-- Add rows from recursive CTE using merge
with C as
(
  select T.ID, T.Parent_Id, T.Topic_Name
  from YourTable as T
  where T.ID = @ID
  union all
  select T.ID, T.Parent_Id, T.Topic_Name
  from YourTable as T
    inner join C 
      on C.ID = T.Parent_Id
)
merge YourTable
using C
on 0 = 1
when not matched then
  insert (Topic_Name) values (C.Topic_Name)
output inserted.ID,
       C.ID,
       C.Parent_Id
  into @T(New_ID, Old_ID, Old_ParentID);

-- Update Parent_Id for the new rows
update Y set
  Parent_Id = T2.New_ID
from @T as T1
  inner join @T as T2
    on T1.Old_ParentID = T2.Old_ID
  inner join YourTable as Y
    on T1.New_ID = Y.ID;

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