Updating a table using Common Table Expression

It is possible to update a table but only the rows returned by the common table expression are modified.

In the Employees table we consider two employees, 'Andrew Fuller' who lives in the city 'Tacoma' and Steven Buchanan who lives in 'London'. Note that the CTE definition has a filter for City='London'

This update query tries to change the LastName to 'Bardick' of an employee whose  FirstName='Andrew'
------------------
USE Northwind
Go
---define the common table expression
WITH EMp_CTE (FirstName,LastName,CITY)  /*Name: EMp_CTE, Column List:FirstName, --LastName,City*/
AS
---define the CTE query

(SELECT FirstName,LastName,City

from Employees

where City='London'
)
---Updating the table
Update EMp_CTE
Set LastName='Bardick'
Where FirstName='Andrew' 
Go
-----
--Andrew Fuller is from Seattle
--expect Andrew Fuller to become Andrew Bardick if the query were to succeed
--Since Andrew Fuller is not from London the table does not get updated
The above query returns '0' rows Table is not modified
-------------------
Now consider this next update query:

USE Northwind
Go
---define the common table expression
WITH EMp_CTE (FirstName,LastName,CITY)  /*Name: EMp_CTE, Column List:FirstName, --LastName,City*/
AS

---define the CTE query

(SELECT FirstName,LastName,City

from Employees

where City='London'
)
---Updating the table

Update EMp_CTE
Set LastName='Bardick'
Where FirstName='Steven' 
Go
--------------------------------------
--The response is 1 row gets updated.
--Steven Buchanan is from London                         
--expect Steven Buchanan to become Steven Buchanan if the query succeeds
--Since Steven Buchanan is from 'London' update will take place and it becomes Steven Bardick as --shown.

The syntax for the CTE can be found here:
Syntax here:
https://msdn.microsoft.com/en-us/library/ms177523.aspx

Stored Procedure using Common Table Expression

You can use the Common Table Expression (CTE) to create a Stored procedure. While CTE is only in memory the Stored Procedure is persisted.
In order to create a stored procedure I will be using the same Common Table Expression used in my two earlier posts here and here.

Run the following in the query pane of SQL Server Management Studio (herein SSMS 2012).
----
USE Northwind
GO
CREATE PROCEDURE dbo.GetName
@city varchar(10)
AS
WITH NCTE(FirstName, LastName,City)
  as

 (SELECT FirstName,LastName,City
 FROM Employees
 )
SELECT *
FROM  NCTE
WHERE city=@city
-----
After the statement is run you will find the stored procedure as shown (you may have to refresh the stored procedures node in the Object Explorer:

---
CTEStoredProc_00

In order to run the stored procedure execute the following statement in the query pane:
Exec GetName 'London'

You get the following query response:

CTEStoredProc_01

Create a view using Common Table Expression

You can use CTE to create a view. Although CTE is in memory, the View will be persisted and can be found in the VIEWS folder in Object Explorer.

I will use the same CTE that I used in the previous post to create the view.
Run this script in the query pane of SQL Server Management Studio (herein SQL Server Developer 2012).

USE Northwind
Go
---define common table expression
Create view CityFolk
as
WITH NEmp_CTE (FirstName,LastName,CITY)  /*Name: Nemp_CTE Column List:FirstName, LastName,City*/
AS
---define the CTE query
(SELECT FirstName,LastName,City
from Employees
where City in ('Redmond','London')
)
Select * from NEmp_CTE
go

You will get the 'Command(s) completed successfully response.

Now you can find the view CityFolk in the VIEWS folder as shown.

CTEView

Common Table Expression (CTE) simply explained

Microsoft documentation defines Common Table Expression thus,

"A common table expression (CTE) 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 is similar to a derived table in that it is not stored as an object and lasts only for the duration of the query. Unlike a derived table, a CTE can be self-referencing and can be referenced multiple times in the same query.

It is unlike a temporary table (persisted) during a connection or a view persisted in the database. CTE is only in memory.

We will consider CTE's many use case scenarios in later posts and the objective of this post is to give a simple example using the Northwind database.

A CTE should have the following:

• CTE with a CTE Expression name
• An Optional Column list
• A query defining the CTE

After a CTE is defined it can be referenced like a table or view and you can do all of the following statements:

• Select
• Insert
• Update
• Delete

It can also be used in defining a view by providing the columns.

Now let me define CTE using the Employees table in Northwind. The example is just to show the syntax as the result can be obtained from a single query without using CTE.

USE Northwind
Go
---define the common table expression
WITH NEmp_CTE (FirstName,LastName,CITY)  /*Name: Nemp_CTE, Column List:FirstName, LastName,City*/
AS
---define the CTE query
(SELECT FirstName,LastName,City
from Employees
where City in ('Tacoma','London')
)
---Use it in a Select Query
SELECT LastName,FirstName,CITY
From NEmp_CTE
where LastName <>'Buchanan'
go

The response to this query is:

Note: The response such as the above does not really need a Common Table Expression but the point was to show the construction of the Syntax used for CTE. In fact, it is to make complex queries more readable.