The WHERE clause subsets the left join to include only those rows containing the value TA. 
   title 'Most Current Information for Ticket Agents';
   select p.IdNumber,
          coalesce(p2.jobcode,p.jobcode) label='Current Jobcode',
          coalesce(p2.salary,p.salary) label='Current Salary'
      from proclib.payroll p left join proclib.payroll2 p2
      on p.IdNumber=p2.idnum
      where p2.jobcode contains 'TA';