Question

I’m trying to select rows one by one in a while loop in SQL Server. And I’m using the only way I know to select the next row using ROW_NUMBER function. But I also have to assign values to t-sql variables in the same SELECT statement thus I’m getting the following error;

A SELECT statement that assigns a value to a variable must not
be combined with data-retrieval operations.

Anyway my code is like this;

   WHILE (@i < 5)
   BEGIN
        SELECT -- This is where the error occurs
            @resultId = video.id, -- this is the value assigning i need to do
            ROW_NUMBER() OVER (ORDER BY dateAdded DESC) AS ROWID
        FROM videoTest.dbo.video
        LEFT JOIN videoTest.dbo.aspnet_Users
        ON video.userId=aspnet_Users.UserId
        WHERE aspnet_Users.UserName=@searchUserName AND ROWID = @i

   -- Processing @resultId       
        SELECT
            compilationId
        FROM videoTest.dbo.comp
        WHERE vidId = @resultId -- i need the id from above
        ....
        ....
        ..... 
        ......

   @i = @i + 1
   END

I would be much appreciated if you can show me way to select next in an alternative way without getting this error, or a way to solve this error in my current code.
Thanks.

Answer 1

You need to separate out the selection of the data from the retrieval of that data into a variable. You could use e.g. a CTE (Common Table Expression) to set up the data, and then operate on that data. However, on a more basic level : why are you assigning the id to @resultId five times?? You don’t seem to be doing anything with the @resultId in the meantime…..

Could you change your logic to be more set-based ? Instead of a WHILE construct – just select the appropriate values from the CTE:

;WITH VideoData AS
(
    SELECT 
        video.id, 
        ROW_NUMBER() OVER (ORDER BY dateAdded DESC) AS ROWID
    FROM 
        videoTest.dbo.video
    LEFT JOIN 
        videoTest.dbo.aspnet_Users ON video.userId = aspnet_Users.UserId
    WHERE 
        aspnet_Users.UserName = @searchUserName 
)
SELECT id, ROWID
FROM VideoData
WHERE ROWID <= 5

Update: if you need the five video.id values for later processing, try something like this:

DECLARE @VideoIDs TABLE (VideoID INT)

;WITH VideoData AS
(
   ... (same as before)
)
INSERT INTO @VideoIDs(VideoID)
   SELECT id
   FROM VideoData
   WHERE ROWID <= 5

-- use your values in the table variable here.....