If I have a table for Mechanics, which could be any mechanic in the US., with each one doing many types of work on cars. I take this to be a many to many relationship [MechId from registered 'Mechanics' table and WorkId from a 'Workcategory' table]

MechID | WorkID

But what if I want to include one other attribute, say list whether or not certified for each type of work by adding a column after WorkID?

Should this table now have a primary index column?