Normalize End Date - Nulls or separate table
When the supplier 'comes back' then do you want to set the EndDate to null again or do you want to keep track of the periods in which the supplier supplied to you? In the last case you must put the StartDate and EndDate in a separate table. If you put it in a separate table then you always must do a join to see if it is an active supplier unless you put an active Y/N switch in the supplier row.