ON clause of JOIN specific syntax?
Normally, you'd simply alias the subquery (which I thought was required in SQL Server)

SELECT DISTINCT ssn, job, identifier, startdate, enddate, salary, ceasing
  FROM oldcontracts
       JOIN (SELECT DISTINCT name, identifier 
               FROM jobs) mysubquery
         ON oldcontracts.job = mysubquery.name


I would strongly suggest that you always alias any tables in your query and that you use those aliases for every column in the select list.  That makes it much easier to see where the different data elements are coming from and makes it easier when you add additional joins in the future.