SQL查询在应该有数值的联接后返回null值

Question

我正在将现有表中的一些数据插入到新表中。有一个名为DescNitro的特定列，它在新表中以0或Null形式返回。我检查了特定的情况，Null值正在替换旧表上的数值。知道为什么会发生这种事吗？以下代码：

DECLARE @DataAtual date = '2021-11-01'; 
DECLARE @UltMes date = DATEADD(MONTH, -1, @DataAtual);
DECLARE @PenUltMes date = DATEADD(MONTH, -2, @DataAtual);
DECLARE @AnoUltMes integer = YEAR(@UltMes);
DECLARE @MesUltMes integer = MONTH(@UltMes);
DECLARE @AnoPenUltMes integer = YEAR(@PenUltMes);
DECLARE @MesPenUltMes integer = MONTH(@PenUltMes);
SELECT
m.datAno
,m.datMes
,m.Segmento
,m.nomDiretoria
,m.nomFilial
,m.Ov
,o.[Região] AS Filial_Pri
,m.[Cod_Rede] AS Cod_Rede
,m.Desc_Rede
,m.CodSegTatica AS cod_Canal_SAP
,m.Canal_Pricing AS cod_Canal_Pri
,m.Des_Canal_Pricing
,m.codMaterial
,m.Vol_Meta AS Vol_Meta_Ciex
,m.Fat_Meta AS Fat_Meta_Ciex
,g.[DescNitro]
INTO
[pri].[tblMetaPrecoteste]
FROM
[pri].[tblDadMetaCiex] m
LEFT JOIN
dbo.tblCadMaterial mat ON mat.codMaterial = m.codMaterial
LEFT JOIN
[pri].[tblDadOvFilial] o ON o.[Organização] = m.Ov
LEFT JOIN 
[pri].[tblDadArvore] a ON a.Material = m.codMaterial
AND a.[Filial] = o.[Região]
AND a.[Canal de Vendas] = m.[Des_Canal_Pricing]
LEFT JOIN
(SELECT
t.[codMaterial], 
t.[codOrganizacaoVenda], 
t.[codSegTatica], 
t.[codRede], 
SUM(t.[DescNitro]) AS DescNitro
FROM
[MktIntelligence].[dbo].[tbl_F_PM_join_real_fifo_meta_com_gp] t
WHERE
(t.datmes = @MesUltMes AND t.datAno = @AnoUltMes) 
OR (t.datmes =  @MesPenUltMes AND t.datAno =  @AnoPenUltMes)  
GROUP BY
t.[codMaterial], t.[codOrganizacaoVenda], t.[codSegTatica], t.[codRede]) g ON m.[codMaterial] = g.[codMaterial]
AND m.[Ov] = g.[codOrganizacaoVenda]
AND m.[codSegTatica] = g.[codSegTatica]
AND m.[Cod_Rede]  = g.[codRede]

Answer 1

由于这是一个left join，因此如果没有匹配，它将显示为NULL，否则使用isnull(columnname,0)，因此当它为NULL 时将显示0