Classification Table – Part 1

joel — Wed, 08 Aug 2007 22:21:46 +0000

Frequently our team is tasked with creating a way to categorize data. One very common and flexible approach is to create a table which contains a series of parent-child relationships. This hierarchical approach can be extremely flexible and efficient for categorizing multiple levels of content. Here is an example of a table used to store this type of classification.

Id	ParentId	Name
1	1	Attributes
2	2	Color
3	2	Red
4	2	Blue
5	2	Green
6	2	Purple
7	1	Font
8	7	Times New Roman
9	8	Bold
10	8	Italic
11	7	Arial
12	11	Bold
13	7	Calibri
14	13	Bold

From this table we can visualize a simple nested tree of classifications

Attributes
- Color
  - Red
  - Blue
  - Green
  - Purple
- Font
  - Times New Roman
    - Bold
    - Italic
  - Arial
    - Bold
    - Calibri

From a SQL perspective, interacting with this structure is quite painless. Through the use of a composite table, this structure lends itself well to having entities with multiple classification.

ClassificationId	EntityId
4	1
9	1
3	2
10	2

If a user wants entities which are classified as “Times New Roman” “Bold” they restrict the results to those with a ClassificationId = 9.

Now what if they want are those entities which have any of the “Font” sub classifications (ClassificationId = 7,8,9,10,11,12,13,14)?

This is where it gets a bit more tricky. The approach I have found most flexible is to create a custom table-valued function which recursively walks the table and returns all children of the specified parent. The results of this reqursive query are returned in the form of a temporary named result set, known as a common table expression (CTE). If our desire was to return more than just the classifications Id we could use this CTE as a sub query on an additional query.

ALTER FUNCTION [Resource].[ClassificationChildren]
(
 @ClassificationId int
)
RETURNS TABLE
AS
RETURN
( WITH Classes (Id, ParentId, [Level]) AS
 (
  -- Create the anchor query. This establishes the starting point
  SELECT c.[Id],c.[ParentId],0
  FROM Classification c
  WHERE c.[Id] = @CategoryId
  UNION ALL
  -- Create the recursive query. This query will be executed
  -- until it returns no more rows
  SELECT
  c.[Id],c.[ParentId], b.[Level]+1
  FROM Classification c
  INNER JOIN Classes b ON COALESCE(c.[ParentId],0) = b.[Id]
 )
SELECT k.Id FROM Classes k
)

Once I have this function, I can create a query to give me the results I desire.

SELECT * FROM Entity LEFT JOIN 
EntryClassification ON Entity.Id = EntryClassification.EntityId
WHERE EntryClassification.ClassificationId in 
(SELECT Id FROM ClassificationChildren(7))

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Classification Table – Part 1