with hoge as (
    SELECT 
        TOTAL_VALUE,
        row_number() over (order by TOTAL_VALUE) as RANK,
         (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 hoge
where RANK between ct/2.0 and ct/2.0 + 1