FIND_IN_SET()
can only be used to search for a single value in a comma-separated list, it doesn't work with two lists.
You'll need to call it separately for each value.
SELECT * FROM tablename
WHERE FIND_IN_SET('12', category_id) OR FIND_IN_SET('13', category_id) OR FIND_IN_SET('15', category_id)
It would be better if you normalized your schema instead of using comma-separated lists. If you create a many-to-many table with the category IDs, you could do:
SELECT t1.*
FROM tablename AS t1
JOIN item_categories AS c ON t1.id = c.table_id
WHERE c.category_id IN (12, 13, 15)
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