Common table expression can be thought of as a temporary result set that is defined within the execution scope of a single SELECT, INSERT, UPDATE, DELETE, or CREATE VIEW statement.
A CTE can be used to:
- Create a recursive query.
- Substitute for a view when the general use of a view is not required; that is, you do not have to store the definition in metadata.
- Enable grouping by a column that is derived from a scalar subselect, or a function that is either not deterministic or has external access.
- Reference the resulting table multiple times in the same statement.
The following example shows the components of the CTE structure: expression name, column list, and query. The CTE expression Sales_CTE has three columns (SalesPersonID, SalesOrderID, and OrderDate) and is defined as the total number of sales orders per year for each salesperson.
USE AdventureWorks2008R2;
GO
-- Define the CTE expression name and column list.
WITH Sales_CTE (SalesPersonID, SalesOrderID, SalesYear)
AS
-- Define the CTE query.
(
SELECT SalesPersonID, SalesOrderID, YEAR(OrderDate) AS SalesYear
FROM Sales.SalesOrderHeader
WHERE SalesPersonID IS NOT NULL
)
-- Define the outer query referencing the CTE name.
SELECT SalesPersonID, COUNT(SalesOrderID) AS TotalSales, SalesYear
FROM Sales_CTE
GROUP BY SalesYear, SalesPersonID
ORDER BY SalesPersonID, SalesYear;
GO
Ref: http://technet.microsoft.com/en-us/library/ms190766(v=sql.105).aspx
