Question

I have implemented a linked list as a self-referencing database table:

CREATE TABLE LinkedList(     Id bigint NOT NULL,     ParentId bigint NULL,     SomeData nvarchar(50) NOT NULL)  

where Id is the primary key, and ParentId is the Id of the previous node on the list. The first node has ParentId = NULL.

I now want to SELECT from the table, sorting the rows in the same order they should appear, as nodes on the list.

Eg.: if the table contains the rows

Id      ParentId  SomeData 24971   NULL      0 38324   24971     1 60088   60089     3 60089   38324     2 61039   61497     5 61497   60088     4 109397  109831    7 109831  61039     6 

Then sorting it, using the criteria, should result in:

Id      ParentId  SomeData 24971   NULL      0 38324   24971     1 60089   38324     2 60088   60089     3 61497   60088     4 61039   61497     5 109831  61039     6 109397  109831    7 

You’re supposed to use the SomeData colum as a control, so please don’t cheat doing ORDER by SomeData 🙂

Answer 1

In Oracle:

SELECT Id, ParentId, SomeData FROM (   SELECT ll.*, level AS lvl   FROM LinkedList ll   START WITH     ParentID IS NULL   CONNECT BY     ParentId = PRIOR Id ) ORDER BY   lvl 

P. S. It’s a bad practice to use NULL as ParentID, as it is not searchable by indices. Insert a surrogate root with id of 0 or -1 instead, and use START WITH ParentID = 0.