Join a table to itself

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 creat

Answer 1

No, you can do self join in your table, there will not be any problem. Are you talking which types of problems in insert, update, delete operation ? You can check some conditions like ParentId exists before adding new record, or you can check it any child exist while deleting parent.

You can do self join like :

select t1.Title, t2.Title as 'ParentName'
from table t1 
left join table t2 
on t1.ParentId = t2.Id  

Answer 2

It is fine to do this (it's a not uncommon pattern). You must ensure that you are adding a child record to a parent record that actually exists etc., but there's noting different here from any other constraint.

You may want to look at recursive common table expressions:

http://msdn.microsoft.com/en-us/library/ms186243.aspx

As a way of querying an entire 'tree' of records.

Answer 3

Its called Self Join and it can be added to a table as in following example

select e1.emp_name 'manager',e2.emp_name 'employee'
from employees e1 join employees e2
on e1.emp_id=e2.emp_manager_id 

Answer 4

I have seen this done without errors before on a table for menu hierarchy you shouldnt have any issues providing your insert / update / delete queries are well written.

For instance when you insert check a parent id exists, when you delete check you delete all children too if this action is appropriate or do not allow deletion of items that have children.

Answer 5

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

Answer 6

You've got plenty of good answers here. One other thing to consider is referential integrity. You can have a foreign key on a table that points to another column in the same table. Observe:

CREATE TABLE tempdb.dbo.t
(
    Id INT NOT NULL ,
    CONSTRAINT PK_t PRIMARY KEY CLUSTERED ( Id ) ,
    ParentId INT NULL ,
    CONSTRAINT FK_ParentId FOREIGN KEY ( ParentId ) REFERENCES tempdb.dbo.t ( Id )
)

By doing this, you ensure that you're not going to get garbage in the ParentId column.