I'd like to use the “nested set model” but I'm obliged to have a GUID as primary key. How can I do without integers as pk?

Question

I'm not aware how deep my tree will be. So I think the NSM is fit for me, reading some docs. In sql, this model suppose I'm using an integer value as primary key. I thought to create a twin table only to store the ints (PK,left,righ) connected by a relation one-to-one with the real table. Things are complicating and it is a waste of space disk, especially when the server is not mine and I have to pay each megabyte. Help!!

UPDATE

Excellent! Fabolous!! Thanks Macka and Bill, I could skip reading a whole book, for now. Celko is a future order on Amazon. ;-)

Answer 1

It doesn't matter what type your primary key is as the left/right values will still be integers. eg.

CREATE TABLE [dbo].[Demo](
    [ID] [uniqueidentifier] ROWGUIDCOL  NOT NULL CONSTRAINT [DF_Demo_ID]  DEFAULT (newid()),
    [Name] [varchar](50) NOT NULL,
    [Lft] [int] NOT NULL,
    [Rgt] [int] NOT NULL,
 CONSTRAINT [PK_Demo] PRIMARY KEY CLUSTERED 
(
    [ID] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]

--add some test data

INSERT INTO demo(name,lft,rgt)
SELECT 'node1',1,6
UNION
SELECT 'node2a',2,3
UNION
SELECT 'node2b',4,5

--check it works

SELECT *
FROM demo
WHERE lft>=2
ORDER BY lft

Answer 2

As @Macka writes, the left and right values are not foreign keys to tree nodes, so they don't have to be the same type. They can be integers while the tree node primary key is a GUID.

Celko also wrote "Trees and Hierarchies in SQL for Smarties" which goes into more detail about Nested Set Model, and other solutions. Reading this book will save you a lot of time and a lot of mistakes.

There are other solutions to storing hierarchical data in a database. See my answer here: What is the most efficient/elegant way to parse a flat table into a tree?