Question

I’ve a beginner question.

This is a piece of code that works:

declare @RowNr int = 1
declare @CA int = 0

While @RowNr <= 1000
BEGIN
    With CCWithRow AS
    (
        SELECT CA ,ROW_NUMBER() OVER (order by CA) as RowNr
        FROM myCATable
    )

    SELECT @CA = CA, @RowNr = RowNr
    FROM CCWithRow
    WHERE RowNr = @RowNr
    Set @RowNr +=  1

--Doing something with @CA here
END

This code doesn’t:

    declare @RowNr int = 1
    declare @CA int = 0

    With CCWithRow AS
    (
            SELECT CA ,ROW_NUMBER() OVER (order by CA) as RowNr
        FROM myCATable
    )

    While @RowNr <= 1000
    BEGIN
        SELECT @CA = CA, @RowNr = RowNr
        FROM CCWithRow
        WHERE RowNr = @RowNr
        Set @RowNr +=  1    
--Doing something with @CA here
    END

Question: Why do I have to “define” the WITH inside the loop?

I know that there are better ways to solve the original problem, so that shouldn’t be the subject. Just want to know, why I can’t define WITH outside of the loop and use it inside.

Answer 1

From BOL

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.

So the common table expression usage must always occur immediately after the end of the CTE definition as it is not in scope to reference anywhere else. This case is no exception.