WITH median AS (
SELECT
 *,
 RANK()OVER(ORDER BY TOTAL_VALUE) AS rank
FROM
 CONVENIENCE
WHERE
 SURVEY_YEAR = 2019
AND
 KIND_CODE = '100'
)

SELECT
 ROUND(SUM(TOTAL_VALUE)/2) AS SA_MEDIAN
FROM
 median
WHERE
 rank = 18
OR 
 rank = 19