Question

this is one on my database tables template.

Id int PK
Title nvarchar(10) unique
ParentId int 

This is my question.Is there a problem if i create a relation between “Id” and “ParentId” columns?
(I mean create a relation between a table to itself)
I need some advices about problems that may occur during insert or updater or delete operations at developing step.thanks

Answer 1

You can perfectly join the table with it self.

You should be aware, however, that your design allows you to have multiple levels of hierarchy. Since you are using SQL Server (assuming 2005 or higher), you can have a recursive CTE get your tree structure.

Proof of concept preparation:

declare @YourTable table (id int, parentid int, title varchar(20))

insert into @YourTable values
(1,null, 'root'),
(2,1,    'something'),
(3,1,    'in the way'),
(4,1,    'she moves'),
(5,3,    ''),
(6,null, 'I don''t know'),
(7,6,    'Stick around');

Query 1 – Node Levels:

with cte as (
    select Id, ParentId, Title, 1 level 
    from @YourTable where ParentId is null

    union all

    select yt.Id, yt.ParentId, yt.Title, cte.level + 1
    from @YourTable yt inner join cte on cte.Id = yt.ParentId
)
select cte.*
from cte 
order by level, id, Title