My trigger is failing to create and I cant understand why - mysql

Question

I'm trying to create this trigger and I get this error:
You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'case where artcStackId = new.artcStackId' at line 7
Here is the failing trigger
SET new.artcPublicId =
case
when new.artcCountry = 'US' then concat(91,new.artcStackId)
when new.artcCountry = 'UK' then concat(92,new.artcStackId)
when new.artcCountry = 'CA' then concat(93,new.artcStackId)
else concat(11,new.artcStackId)
end case
where artcStackId = new.artcStackId
Here is the trigger that works
SET new.artcPublicId =
case
when new.artcCountry = 'US' then concat(91,new.artcStackId)
when new.artcCountry = 'UK' then concat(92,new.artcStackId)
when new.artcCountry = 'CA' then concat(93,new.artcStackId)
else concat(11,new.artcStackId)
end
when I add those extra bits, it'll fail.

Answer 1

There is no reference to your where condition.i think it should be like where new.artcStackId = new.artcStackId

Answer 2

Your first trigger has a syntax error:
end case
should be just
end
ie remove the trailing "case"
Edit:
I just noticed you have a where condition: where artcStackId = new.artcStackId.
You can't do that. You can only modify the current row, referenced by the new. syntax.
You must remove the where clause altogether, because you may only update the row being inserted/updated.