为什么我的CASE语句失败？

Question

SELECT c.AssetClass, SUM(c.MarketValue) AS MVCurrent, SUM(p.MarketValue) AS MVPrior, SUM(c.MarketValue) / SUM(p.MarketValue) AS PercentageVariation,
    CASE
    WHEN SUM(c.MarketValue) / SUM(p.MarketValue) BETWEEN 0.9 AND 1.1
    THEN 'PASS'
    ELSE 'FAIL'
    END AS Result
FROM [Nov] AS p, [Dec] AS c
;

是否与双表有关？