WITH subq AS (
    --サブクエリで全レコードの平均値とそれに対応する番号付与
    SELECT
    TOTAL_VALUE
    ,CAST(ROW_NUMBER() OVER (ORDER BY TOTAL_VALUE) AS REAL) AS number
FROM
    CONVENIENCE
WHERE
    SURVEY_YEAR = 2019
    AND KIND_CODE = '100'
    )
SELECT
    ROUND(AVG(TOTAL_VALUE))
FROM
    subq
WHERE
--3種類の平均値を取得
    number IN ((SELECT MAX(NUMBER) / 2 FROM subq)
               ,(SELECT (MAX(NUMBER) / 2) + 1 FROM subq)
               ,(SELECT (MAX(NUMBER) / 2) + 0.5 FROM subq)
               );