SELECT
    ROUND(AVG(TOTAL_VALUE)) AS SA_MEDIAN
FROM
    convenience
WHERE
    survey_year = 2019
    AND kind_code = '100'
ORDER BY
    TOTAL_VALUE
LIMIT CASE
    WHEN (SELECT COUNT(1) % 2 FROM convenience) = 0 THEN 2
    ELSE 1
END
-- n_record: 6 -> 3 ~ 5
-- n_record: 7 -> 4
-- n_record: 8 -> 4~5
OFFSET ( --開始位置(この行は含まない)
    SELECT 
        (COUNT(1) - 1)/ 2 
    FROM 
        convenience
)