Hi @lllqw ,
Maybe also an alternate method would be to use Join Datatables Activity with Left Join. Then update the Column “Attending” using Data Column Expression since it is a relatively simpler updation.
Check the workflow below :
DT_Join_ColumnValues.zip (9.3 KB)
For simplicity purpose, I have used both the data as xlsx, you could use the Read CSV Activity for reading the csv files in your case.
You could also refer the Blog below for more info on Updation of Datatable values :
Let us know if this doesn’t help.