Updating multiple rows using list of Ids

just use the ids as integers like so:

UPDATE mytbl
SET flag = 1
WHERE id in (1,2,3,....)

Try this:

UPDATE mytbl
SET flag = 1
WHERE @IdList LIKE CONCAT('%,',Id,',%');

Note that your varchar list @IdList must start and end with comma (e.g. ,1,2,20,30,).

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