Case when in sql and php -

i have function in php, check, using sql if car garage booked. have 2 types of tables, garages can stored. in original function use first table:

    $fields = "select dd.id, h.id carhireid, h.name carhirename,                       f.course courseid, c.shortname courseshort,                       sd.id sessiondateid, sd.timestart, sd.timefinish, rd.garageid garagedateid, r.name garagename";     $sql = "               {driver_date} dd               join {carhire_sessions_dates} sd on sd.id = td.dateid               join {carhire_sessions} s on s.id = sd.sessionid               join {carhire} h on h.id = s.carhire               join {course} c on c.id = h.course               join {alternativemanagement_garage_date} rd on rd.dateid = td.dateid               left join {carhire_garage} r on r.id = rd.garageid              dd.userid = ?";     $sort = "              order sd.timestart desc, sd.timefinish desc, f.name, td.id"; 

there alternative, have garageid each session in table carhire_sessions.

the names of garages stored in carhire_garage

so want add follows: if roomid in table carhire_sessions null, use alternativemanagement_garage_date