SQL Server 2005 CTE (Common Table Expression)

CTE is a new feature provided by Microsoft SQL Server 2005. In real world, we often need to query hierarchical data from the database. For example, to get a list of hierarchical list of all the employees, list of product categories etc. CTE fulfills this requirement and let us query the database recursively.

Let’s us do this practically. Assume that we store different categories of computer books and any category can have sub-categories. For this, we will create a table named tblCategories with the following structure and insert some categories into this table as shown below:

Create Table tblCategories
(
CategoryID Int Constraint PK_tblCategories_CategoryID Primary Key,
CategoryName VarChar(100),
ParentCategoryID Int Constraint FK_tblCategories_ParentCategoryID References tblCategories(CategoryID)
)

GO

Insert Into tblCategories(CategoryID,CategoryName,ParentCategoryID) Values(1,'Languages',Null)

Insert Into tblCategories(CategoryID,CategoryName,ParentCategoryID) Values(2,'Networking',Null)

Insert Into tblCategories(CategoryID,CategoryName,ParentCategoryID) Values(3,'Databases',Null)

Insert Into tblCategories(CategoryID,CategoryName,ParentCategoryID) Values(4,'Visual Basic',1)

Insert Into tblCategories(CategoryID,CategoryName,ParentCategoryID) Values(5,'C#',1)

Insert Into tblCategories(CategoryID,CategoryName,ParentCategoryID) Values(6,'Java',1)

Insert Into tblCategories(CategoryID,CategoryName,ParentCategoryID) Values(7,'VB.Net',4)

Insert Into tblCategories(CategoryID,CategoryName,ParentCategoryID) Values(8,'VB 6.0',4)

Insert Into tblCategories(CategoryID,CategoryName,ParentCategoryID) Values(9,'Desktop Application Development with VB.Net',7)

Insert Into tblCategories(CategoryID,CategoryName,ParentCategoryID) Values(10,'Web Application Development with VB.Net',7)

Insert Into tblCategories(CategoryID,CategoryName,ParentCategoryID) Values(11,'ActiveX Objects and VB 6.0',8)

Insert Into tblCategories(CategoryID,CategoryName,ParentCategoryID) Values(12,'Network Security',2)

Now if you query the database with the following SELECT command,

Select * From tblCategories Where CategoryID = 1

You will get the following result:

CategoryID CategoryName ParentCategoryID
----------- ---------------- ----------------
1 Languages NULL

(1 row(s) affected)

No surprise! But what, if you want to get the list of all the categories/sub-categories falling under the root category ‘Languages’? To do this, you will need to perform a recursive query and to do that we use CTE.

Let’s do this with the help of CTE. To create CTE we will use the following syntax:

With cteCategories
AS (
Select CategoryID,CategoryName,ParentCategoryID
From tblCategories
Where CategoryID=1

Union All

Select C.CategoryID,C.CategoryName,C.ParentCategoryID
From tblCategories As C Inner Join cteCategories As P On C.ParentCategoryID = P.CategoryID

)

Select CategoryID,CategoryName,ParentCategoryID From cteCategories

Run the above query and see the result as shown below:

CategoryID CategoryName ParentCategoryID
----------- ---------------------------------------------------- ----------------
1 Languages NULL
4 Visual Basic 1
5 C# 1
6 Java 1
7 VB.Net 4
8 VB 6.0 4
11 ActiveX Objects and VB 6.0 8
9 Desktop Application Development with VB.Net 7
10 Web Application Development with VB.Net 7

(9 row(s) affected)