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 avg(TOTAL_VALUE) as SA_MEDIAN
from ordered_purchases
where row_id between ct/2.0 and ct/2.0 + 1;