Monday, May 2, 2016

What is a Common Table Expression (CTE) in SQL Server?

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
---define the common table expression
WITH NEmp_CTE (FirstName,LastName,CITY)  /*Name: Nemp_CTE, Column List:FirstName, LastName,City*/
---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'

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.

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