WITH ORDER_CONVENIENCE AS (
SELECT
  TOTAL_VALUE,
  ROW_NUMBER() OVER (ORDER BY TOTAL_VALUE) AS ROW_ID,
  (SELECT COUNT(1) FROM CONVENIENCE WHERE SURVEY_YEAR = 2019) AS CT
  FROM CONVENIENCE
  WHERE SURVEY_YEAR = 2019
)
SELECT AVG(TOTAL_VALUE) AS SA_MEDIAN
FROM ORDER_CONVENIENCE 
WHERE ROW_ID BETWEEN CT/2.0 AND CT/2.0 + 1;