Need help updating records from a subset yielded by a subquery.

Question

Hi, retired, hobby coder needing (further) help in transact-sql scripting with a view to creating a stored procedure within a Sequel DB.  For the purposes of this investigation I have a table with 23 records.   I am trying to update a specific
field for 10 of the records based on the (TOP 10) records selected from a sub query where I select the TOP 20 records based upon different field.
I select my desired 10 records (TOP 10) from a sup query selecting the TOP 20 based upon the date field (latest 20) for a specific user (GID) as follows:
I then attempt to update only these 10 records as follows:
As you can see it updated all 23 rows in the table, not just the 10 I yielded by the underlying query.
In reviewing the documentation it appeared that referencing the Scores_id column and using the 'IN' keyword would work, i.e., UPDATE Scores WHERE Score_id IN (SELECT TOP 10* ..... would be appropriate but I got the an error message, 'Only one expression
can be specified in the select list when the subquery is not introduced with EXISTS.'
Can someone assist me with the proper syntax for this script?  Additionally, since there are limitations on the number of variables which can be used any advice on how to structure a stored procedure where the GID is the initial variable and Score_id
the second would be appreciated also.
Regards,
Minuend

Answer 1

The query you have written will always update all or no rows in the table, since it says
UPDATE Scores
SET    ...
WHERE  EXISTS (...)
The EXISTS clause is, logically, evaluated for each row. Normally, when you use EXISTS you correlated the subquery with the surrounding query, so that the values the subquery works with are different for each row. But your subquery is uncorrelated, and will
return the same value, TRUE or FALSE, for each row.
IN can certainly work, but then you cannot have "SELECT TOP 10 *", but you need to have "SELECT TOP Score_id", since when you use IN, the subquery must be a single-column query.
Another alternative, which is somewhat difficult to digest the first you see it, but which is more efficient, and more concise, once you have learn it, is to work directly from the query you have:
  WITH CTE AS (
     SELECT TOP 10 *
      FROM (SELECT TOP 20...
 UPDATE CTE
 SET    Used = '*'
Erland Sommarskog, SQL Server MVP, [email protected]