SELECT
    AVG(total_value) AS SA_MEDIAN
FROM (
    SELECT
        total_value
    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
    ) 
) x