--条件内のTOTAL_VALUEを算出し、RANK付けを行う
WITH subq AS (
    SELECT
        CAST(RANK() OVER (ORDER BY TOTAL_VALUE) AS float) AS RANK
        ,TOTAL_VALUE
    FROM
        CONVENIENCE
    WHERE
        SURVEY_YEAR = 2019
        AND KIND_CODE = '100'
    )
--サブクエリで作成したテーブルをもとに中央値を計算
SELECT
    ROUND(AVG(TOTAL_VALUE)) AS SA_MEDIAN
FROM
    subq
WHERE
    RANK IN (--偶数の場合
             (SELECT COUNT(RANK) AS AMT FROM subq) / 2
             ,(SELECT COUNT(RANK) AS AMT FROM subq) / 2 + 1
             --奇数の場合
             ,(SELECT COUNT(RANK) AS AMT FROM subq) / 2 + 0.5);