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Tuesday, November 8, 2011

CTE Common Table Expression

The CTE is one of the essential features in the sql server 2005.It just store the result as temp result set. It can be access like normal table or view. This is only up to that scope.

The syntax of the CTE is the following.

WITH name (Alias name of the retrieve result set fields)
AS
(
//Write the sql query here
)
SELECT * FROM name

Here the select statement must be very next to the CTE. The name is mandatory and the argument is an optional. This can be used to give the alias to the retrieve field of the CTE.

CTE 1: Simple CTE

WITH ProductCTE
AS
(
SELECT ProductID AS [ID],ProductName AS [Name],CategoryID AS [CID],UnitPrice AS [Price]
FROM Products
)
SELECT * FROM ProductCTE

Here all the product details like ID, name, category ID and Unit Price will be retrieved and stored as temporary result set in the ProductCTE.

This result set can be retrieved like table or view.

CTE2:Simple CTE with alias

WITH ProductCTE(ID,Name,Category,Price)
AS
(
SELECT ProductID,ProductName,CategoryID,UnitPrice
FROM Products
)
SELECT * FROM ProductCTE

Here there are four fieds retrieves from the Products and the alias name have given in the arqument to the CTE result set name.

It also accepts like the following as it is in the normal select query.

WITH ProductCTE
AS
(
SELECT ProductID AS [ID],ProductName AS [Name],CategoryID AS [CID],UnitPrice AS [Price]
FROM Products
)
SELECT * FROM ProductCTE

CTE 3: CTE joins with normal table

The result set of the CTE can be joined with any table and also can enforce the relationship with the CTE and other tables.

WITH OrderCustomer
AS
(
SELECT DISTINCT CustomerID FROM Orders
)
SELECT C.CustomerID,C.CompanyName,C.ContactName,C.Address+', '+C.City AS [Address] FROM Customers C INNER JOIN OrderCustomer OC ON OC.CustomerID = C.CustomerID

Here the Ordered Customers will be placed in the CTE result set and it will be joined with the Customers details.

CTE 4: Multiple resultsets in the CTE

WITH MyCTE1
AS
(
SELECT ProductID,SupplierID,CategoryID,UnitPrice,ProductName FROM Products
),

MyCTE2
AS
(
SELECT DISTINCT ProductID FROM "Order Details"
)
SELECT C1.ProductID,C1.ProductName,C1.SupplierID,C1.CategoryID FROM MyCTE1 C1 INNER JOIN MyCTE2 C2 ON C1.ProductID = C2.ProductID

Here, there are two result sets that will be filtered based on the join condition.

CTE 5: Union statements in the CTE

WITH PartProdCateSale
AS
(
SELECT ProductID FROM Products WHERE CategoryID = (SELECT CategoryID FROM Categories WHERE CategoryName='Condiments')
UNION ALL
SELECT ProductID FROM Products WHERE CategoryID = (SELECT CategoryID FROM Categories WHERE CategoryName='Seafood')
)
SELECT OD.ProductID,SUM(OD.UnitPrice*OD.Quantity) AS [Total Sale] FROM "Order Details" OD INNER JOIN PartProdCateSale PPCS ON PPCS.ProductID = OD.ProductID
GROUP BY OD.ProductID

Normally when we combine the many result sets we create table and then insert into that table. But see here, we have combined with the union all and instead of table, here CTE has used.

CTE 6: CTE with identity column

WITH MyCustomCTE
AS
(
SELECT CustomerID,row_number() OVER (ORDER BY CustomerID) AS iNo FROM
Customers
)
SELECT * FROM MyCustomCTE

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