SELECT round(Avg(V)) AS SA_MEDIAN
FROM (SELECT Max(TOTAL_VALUE) AS V
      FROM (SELECT CONVENIENCE.TOTAL_VALUE
            FROM CONVENIENCE 
            where SURVEY_YEAR = 2019
                and KIND_CODE = '100'
            ORDER BY TOTAL_VALUE
            limit 18)
      UNION ALL
      SELECT Min(TOTAL_VALUE)
      FROM (SELECT CONVENIENCE.TOTAL_VALUE
            FROM CONVENIENCE 
            where SURVEY_YEAR = 2019
                and KIND_CODE = '100'
            ORDER BY TOTAL_VALUE DESC
            limit 18));