SELECT 
Jobs.Job, 
Users.User, 
Count(Users.User) AS CountOfUser

FROM Jobs LEFT JOIN Users 
     ON (
          (
              Jobs.Start <= Users.Start and 
              Jobs.Stop >= Users.Start
                                              ) or 
           (
              Jobs.Start <= Users.Stop and 
              Jobs.Stop >= Users.Stop
                                              ) or
           (
              Users.Start <= Jobs.Start and 
              Users.Stop >= Jobs.Start
                                              ) or
           (
              Users.Start <= Jobs.Stop and
              Users.Stop >= Jobs.Stop         )
       )

GROUP BY Jobs.Job, Users.User

HAVING Count(Users.User) = 1;