这是[具有可接受的性能]计算比率的正确方法：

With AnnualQuotaCTE as (
    SELECT
        SUM(UnitPrice) as TotalUnitPrice,UnitNo
    FROM MonthlyUnitPrice
    WHERE [Year] = '1398'
    Group by UnitNo

),TotalUsageCTE as (
    SELECT
        SUM(TotalCalculatedPrice) as SumTotalCalculatedPrice,UnitNo
    FROM [ManagerTable]
    WHERE [Year] = '1398' AND [Month] < '9'
    Group by UnitNo
)

SELECT 
    MT.UnitNo,UC.UnitName,AnnualQuotaCTE.AnnualQuota,TotalUsageCTE.TotalUsage,(Isnull(AnnualQuotaCTE.AnnualQuota,0) - isnull(TotalUsageCTE.TotalUsage,0)) as UsageRatio
FROM ManagerTable MT
INNER JOIN UnitCodes UC ON UC.UnitNO = MT.UnitNO
INNER JOIN MonthlyUnitPrice MU ON MT.UnitNO = MU.UnitNO AND MT.[Year] = MU.[Year] AND MT.[Month] = MU.[Month]
LEFT JOIN AnnualQuotaCTE on AnnualQuotaCTE.UnitNo = MT.UnitNo
LEFT JOIN TotalUsageCTE on TotalUsageCTE.UnitNo = MT.UnitNo
WHERE MT.[Year] = '1398' AND MT.[Month] < 9 AND MU.UnitPrice IS NOT NULL
GROUP BY MT.UnitNo,TotalUsageCTE.TotalUsage

此外，您可以将年和月添加到CTE和主要查询中。

我认为您可以大大简化查询：

SELECT MT.UnitNo,SUM(UnitPrice) as TotalUsage,AnnualQuota
       (AnnualQuota - SUM(UnitPrice)
FROM ManagerTable MT JOIN
     UnitCodes UC 
     ON UC.UnitNO = MT.UnitNO LEFT JOIN
     (SELECT MUP.UnitNo,SUM(UnitPrice) AS AnnualQuota
      FROM MonthlyUnitPrice MUP
      WHERE MUP.[Year] = '1398'
      GROUP BY MUP.UnitNo
     ) MUP
     ON MUP.UnitNo = MT.UnitNo
WHERE MT.[Year] = '1398' AND
      MT.[Month] < 9
GROUP BY MT.UnitNo,UC.UnitName;

如果您需要转换为BIGINT：

SELECT MT.UnitNo,SUM(CONVERT(BIGINT,UnitPrice)) as TotalUsage,AnnualQuota
       (AnnualQuota - SUM(CONVERT(BIGINT,UnitPrice))
FROM ManagerTable MT JOIN
     UnitCodes UC 
     ON UC.UnitNO = MT.UnitNO LEFT JOIN
     (SELECT MUP.UnitNo,UnitPrice)) AS AnnualQuota
      FROM MonthlyUnitPrice MUP
      WHERE MUP.[Year] = '1398'
      GROUP BY MUP.UnitNo
     ) MUP
     ON MUP.UnitNo = MT.UnitNo
WHERE MT.[Year] = '1398' AND
      MT.[Month] < 9
GROUP BY MT.UnitNo,UC.UnitName