Scope of an CTE in SQL Server 2005

Question

WITH emp_CTE AS (
    SELECT ROW_NUMBER() OVER(ORDER BY (SELECT 1)) AS IdentityId, *
    FROM dbo.employee )
SELECT * FROM emp_CTE

This works fine<

Answer 1

The CTE is part of the subsequent statement only.

The subsequent statement can be a single SELECT/INSERT/UPDATE/DELETE, or a compound (with UNION, INTERSECT etc)

For example:

;WITH cte1 AS
(
   select ...
), cte2 AS
(
    select ...
)
SELECT ...
UNION 
SELECT ...;

The rule of thumb is that the scope is until where next ; would be. A semi-colon terminates any statement but is optional unfortunately.

Your failing code above is actually this

...;
WITH emp_CTE AS (
    SELECT ROW_NUMBER() OVER(ORDER BY (SELECT 1)) AS IdentityId, *
    FROM dbo.employee )
SELECT * FROM EMPLOYEES; 
SELECT * FROM emp_CTE;

So the CTE is only in scope up until ...EMPLOYEES;

Answer 2

In short: NO. A CTE is valid exactly for the single, next statement - no more. And there's no way to "extend" the life of a CTE either.

From MSDN Books Online:

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.

You can:

• turn your CTE into a view and use that view for your queries - views are available as long as they are defined

• store the results of the CTE into a temporary table or table variable for further processing

• change your statements after the CTE so they can be executed as a single statement

  WITH emp_CTE AS (
  ......)
  SELECT (list of columns)
  FROM emp_CTE
  INNER JOIN dbo.Employees e ON ......