with ordered_purchases as (
  select
      TOTAL_VALUE,
      row_number() over (order by TOTAL_VALUE) as row_id,
      (select count(1) from CONVENIENCE) as ct
  from CONVENIENCE
  where SURVEY_YEAR = 2019
  and KIND_CODE = '100'
)
select round(avg(TOTAL_VALUE)) as SA_MEDIAN
from ordered_purchases
where row_id between ct/2.0 and ct/2.0 + 1;