select avg(total_value)
from 
  (SELECT total_value
   FROM convenience
   where survey_year = 2019
     and kind_code = '100'
   ORDER BY total_value
   LIMIT 2 - (SELECT COUNT(*) FROM convenience) % 2    -- odd 1, even 2
   OFFSET (SELECT (COUNT(*) - 1) / 2 FROM convenience))
;