连接多个表将返回NULL值

Question

我有三种不同的CTE结果,我需要互相左右联合:

主表@Policies包含所有VehiclePolicyLimitsID值:

@LiabilityPremium:

@HiredPremium:

作为一个例子,我将CTE的结果模仿成3个表变量:

declare @Policies table (VehiclePolicyLimitsID int)
insert into @Policies values (2101891),
                             (2101892),
                             (2101893),
                             (2101894),
                             (2119235),
                             (2119236),
                             (2119237),
                             (2119238),
                             (2190860),
                             (2190861),
                             (2190862),
                             (2190863)
--select * from @Policies


declare @LiabilityPremium  table (Quoteid int, ClassCode int, VehiclePolicyLimitsID int, LiabilityPremium money)
insert into @LiabilityPremium values (728436,3199,2101892,1723),
                                     (728436, 23199,2101893,1855),
                                     (728436,68199,2101894,133),
                                     (741626,3199,2119236,0),
                                     (741626,23199,2119237,0),
                                     (741626,68199,2119238,0),
                                     (774168,3199,2190861,0),
                                     (774168,23199,2190862,0),
                                     (774168,68199,2190863,0)
--select * from @LiabilityPremium

declare @HiredPremium  table (Quoteid int, ClassCode int, VehiclePolicyLimitsID int, LiabilityPremium money)
                    insert into @HiredPremium values ( 728436,  NULL,   2101891,    25),
                                                     (741626,   NULL,   2119235,    0),
                                                     (774168,   NULL,   2190860,    0)

--select * from @HiredPremium
    select 
            COALESCE(l.Quoteid,h.QuoteID,'') as QuoteID,
            COALESCE(l.ClassCode,h.ClassCode,'') as ClassCode,
            COALESCE(l.VehiclePolicyLimitsID,h.VehiclePolicyLimitsID,'') as VehiclePolicyLimitsID,
            l.LiabilityPremium + h.LiabilityPremium as LiabilityPremium
    from @Policies p
    left join @LiabilityPremium l ON l.VehiclePolicyLimitsID = p.VehiclePolicyLimitsID
    left join @HiredPremium h ON h.VehiclePolicyLimitsID = p.VehiclePolicyLimitsID

但由于某种原因,LiabilityPremium的结果都是NULL:

我希望结果看起来像这样,总LiabilityPremium = $ 3,736

有什么办法以某种方式加入以获得理想的结果？

Answer 1

这是因为null加法运算符的任一侧都会产生 的结果null。您可以使用ISNULL(LiabilityPremium, 0)示例：

ISNULL(l.LiabilityPremium,0) + ISNULL(h.LiabilityPremium,0) as LiabilityPremium

或者您可以使用COALESCE代替ISNULL.

COALESCE(l.LiabilityPremium,0) + COALESCE(h.LiabilityPremium,0) as LiabilityPremium

编辑

我不确定这是否与这个小数据集一致或符合预期，但如果总是预期@LiabilityPremium.LiabilityPremium或@HiredPremium.LiabilityPremium始终为空，则无需执行加法。COALESCE相反，直接在这两列上使用。

COALESCE(l.LiabilityPremium, h.LiabilityPremium) as LiabilityPremium