尝试像下面这样使用 group_concat() 你可以得到 , 分隔的列表，然后在它们之间加入桌子.但是存储逗号分隔值从来都不是一个好主意.

Try like below by using group_concat() you can get the , separated list and then join between the tables. But storing comma separated values is never a good idea.

update table2 t2
join
(
select ItemID,group_concat(CategoryID) as newcat
from table1 group by ItemID
) tab on t2.ItemID = tab.ItemID        
set t2.CatIDs = tab.newcat