About mysql problem

[somequestion]

I'd like to make format string when table is updated.

there are 2 table member and item.

member table
-memberId
-PurchasedItems

item table
-itemId
-memberId

if query is like below i want to add format string like "1,2" to itemList in member table.
so how can i change this?

SELECT distinct itemId FROM `item` WHERE memberId = 1123
// item 1,2

and also make it trigger to update automatically.
how can i change this too??

CREATE TRIGGER `member_item_Trigger` BEFORE INSERT ON `item`
 FOR EACH ROW BEGIN
 
UPDATE member SET itemList = (SELECT distinct itemId FROM item WHERE memberId = NEW.memberId)  WHERE memberId = New.memberId;

END;