Hierarchies in SQL Server 2008

最近在做一个silverlight项目，有关于树的，需要用到SQL Server 2008里的Hierarchies ，以下文章是转载自国外的一个博客，自感觉挺不错的：

Graphs, tree algorithms and structures have been used for long time in databases
to solve hierarchy related problems. Adjacency list, nested sets, materialized
path, and other hybrid methods offer different capabilities to help.

SQL
Server 2008 adds a new feature to help with modeling hierarchical relationships:
the HIERARCHYID data type. It provides compact storage and convenient methods to
manipulate hierarchies. In a way it is very much like optimized materialized
path. In addition the SqlHierarchyId CLR data type is available for client
applications.

While HIERARCHYID has a lot to offer in terms of
operations with hierarchical data, it is important to understand a few basic
concepts:

- HIERARCHYID can have only a single root (although easy to
work around by adding sub-roots)

- It does not automatically represent a
tree, the application has to define the relationships and enforce all rules

- The application needs to maintain the consistency

Here is a one
example of employee hierarchy to illustrate the usage of HIERARCHYID and the
related methods for manipulation of hierarchies.

CREATE TABLE

Employees (

 emp_id INT

NOT NULL

PRIMARY KEY

,

 emp_name VARCHAR

(35),

 manager_id INT REFERENCES

Employees(emp_id),

 org_chart_path HIERARCHYID

);

 

/*

 

The primary key prevents cyclic paths.

Another way is using a CHECK constraint.

 

CHECK (org_chart_path.ToString() NOT LIKE '%/' + CAST(emp_id AS VARCHAR(10)) + '/_%')

 

*/

 

-- Insert the top level manager as hierarchy root

INSERT INTO

Employees

VALUES

(1, 'Jeff Brown'

, NULL

, hierarchyid

::GetRoot());

 

-- Insert John who reports to the top level manager

INSERT INTO

Employees

VALUES

(2, 'John Doe'

, 1,

           (SELECT

hierarchyid

::GetRoot().GetDescendant(NULL

, NULL

)

            FROM

Employees));

 

 

-- Insert Peter at the same level as John

DECLARE

@mgr HIERARCHYID

= (SELECT

org_chart_path

                            FROM

Employees

                            WHERE

emp_name = 'John Doe'

);

 

INSERT INTO

Employees

VALUES

(3, 'Peter Hanna'

, 1,

           (SELECT

hierarchyid

::GetRoot().GetDescendant(@mgr, NULL

)

            FROM

Employees

            WHERE

org_chart_path = hierarchyid

::GetRoot()));

 

-- Insert Richard as reporting to John

INSERT INTO

Employees

VALUES

(4, 'Richard Burns'

, 2,

           hierarchyid

::Parse('/1/1/'

)); -- Also: CAST('/1/1/' AS HIERARCHYID)

 

SELECT

emp_id, emp_name,

       manager_id, org_chart_p
dde9
ath,

       org_chart_path.GetAncestor(1) AS

emp_manager,

       hierarchyid

::GetRoot() AS

top_manager,

       org_chart_path.GetDescendant(NULL

, NULL

) AS

emp_descendant,

       org_chart_path.GetLevel() AS

emp_level,

       org_chart_path.ToString() AS

emp_org_path

FROM

Employees;

 

/*

 

emp_id emp_name

        manager_id

org_chart_path

emp_manager

  top_manager

emp_descendant

emp_level emp_org_path

------ ---------------- ----------- --------------- ------------- ------------ --------------- --------- ------------

1

    Jeff Brown

      NULL

       0x

            NULL

        0x

          0x58

           0

        /

2

    John Doe

        1

          0x58

           0x

           0x

          0x5AC0

        1

        /1/

3

    Peter Hanna

    1

          0x68

           0x

           0x

          0x6AC0

        1

        /2/

4

    Richard Burns

   2

          0x5AC0

        0x58

        0x

          0x5AD6

        2

        /1/1/

 

*/

 

-- Move Richard to report to Peter

DECLARE

@new_mgr HIERARCHYID

= (SELECT

org_chart_path

                                FROM

Employees

                                WHERE

emp_name = 'Peter Hanna'

);

 

UPDATE

Employees

SET

org_chart_path = org_chart_path.Reparent(org_chart_path.GetAncestor(1),

                                            @new_mgr)

WHERE

emp_name = 'Richard Burns'

;

 

SELECT

emp_id, emp_name,

       manager_id, org_chart_path,

       org_chart_path.GetAncestor(1) AS

emp_manager,

       hierarchyid

::GetRoot() AS

top_manager,

       org_chart_path.GetDescendant(NULL

, NULL

) AS

emp_descendant,

       org_chart_path.GetLevel() AS

emp_level,

       org_chart_path.ToString() AS

emp_org_path

FROM

Employees;   

 

/*

 

emp_id

emp_name

       manager_id

org_chart_path

   emp_manager

  top_manager

emp_descendant

  emp_level emp_org_path

------- --------------- ----------- ----------------- ------------- ------------ ---------------- --------- ------------

1

      Jeff Brown

    NULL

       0x

               NULL

        0x

          0x58

            0

        /

2

      John Doe

       1

          0x58

            0x

           0x

          0x5AC0

          1

        /1/

3

      Peter Hanna

    1

          0x68

            0x

           0x

          0x6AC0

          1

        /2/

4

      Richard Burns

  2

          0x6AC0

           0x68

        0x

          0x6AD6

          2

        /2/1/

 

*/

From
the above example it is very easy to see the similarity between materialized
path and HIERARCHYID when the HIERARCHYID is converted to the character format
using the ToString() method. Converting hierarchy from traditional parent/child
format to HIERARCHYID is simple using recursive CTEs (very similar to building a
materialized path).

Note:

This code has been tested with SQL
Server 2008 CTP 6 (February 2008). As of SQL Server 2008 Release Candidate 0
(June 2008) the method Reparent() has been replaced with the method
GetReparentedValue(). It is called using the same parameters and returns the
same value.