SELECT
	SiteCode,
	SUM( SomeStuff) AS SumOfStuff
FROM
	(SELECT SiteCode, SomeStuff FROM table1
	UNION ALL
	SELECT SiteCode, SomeStuff From Table2
	) AS drv
GROUP By
	SiteCode

WITH BaseData as (
SELECT
    a.PLANT_NO AS PlantNumber,
    SUM(a.APPLIED_VOL) AS AppliedVolume
FROM
    [EdwStaging].[RAW].[FactSettleFee_TIPS_QRMTIPS_QPOST_SETTLE_FEE] a
GROUP BY
    a.PLANT_NO
UNION ALL
SELECT
    CASE WHEN a.plant_no = 'ALL'
        THEN e.[PlantNumber]
        ELSE a.plant_no
    END AS ResolvedPlantNumber,
    SUM(TRANS_VOL) AS AppliedVolume
FROM
    [EdwStaging].[RAW].[FactSettleFee_TIPS_QRMTIPS_QPOST_RPTS_INVOICE_DTL] a
    LEFT OUTER JOIN [EdwStaging].[PSTG].[ContractToPlantMapping] e
        ON a.ctr_no = e.[ContractNumber]
           AND a.PROD_DT BETWEEN e.[PlantEffectiveFromDate] AND e.[PlantEffectiveToDate]
           AND a.PROD_DT BETWEEN e.[ContractEffectiveFromDate] AND e.[ContractEffectiveToDate] 
           AND a.plant_no = 'ALL'
GROUP BY
    CASE WHEN a.plant_no = 'ALL'
        THEN e.[PlantNumber]
        ELSE a.plant_no
    END 
)
select
    PlantNumber,
    sum(AppliedVolume)
from
    BaseData
group by
    PlantNumber;