在from子句之后尝试payable表，并使用left join连接其他表

FROM dbo.Fact_AccountPayables         
LEFT JOIN dbo.Fact_PurchaseOrder ON  dbo.Fact_AccountPayables.[PO Number] =  dbo.Fact_PurchaseOrder.[Purchase Order]   AND 
 dbo.Fact_AccountPayables.[PO Line] =  dbo.Fact_PurchaseOrder.Sequence
LRFT JOIN dbo.Supplier  ON dbo.Supplier.supplier_key = dbo.Fact_PurchaseOrder.buyfrom_supplier_key AND dbo.Fact_AccountPayables.supplier_key = dbo.Supplier.supplier_key

您不仅使用了供应商密钥，还使用了订单号和订单行作为完全外部联接的联接标准。如果在supplier表中没有匹配的条目（同时匹配所有联接条件的条目），您还将看到您提到的空值。
只是为了测试，暂时删除其他连接条件（订单号和行），只留下供应商密钥。检查在这种情况下是否仍然看到空值。

您正在对fact\u accountpayables表使用完全联接。这样，即使与其他两个表不匹配，也会包括来自“应付帐款”的所有记录。如果您不想这样做，请使用左连接而不是完全连接。

SELECT
  ISNULL(dbo.Supplier.supplier_key, dbo.Fact_AccountPayables.supplier_key) AS supplier_key,
  dbo.Supplier.Supplier,
  dbo.Supplier.Name,
  dbo.Supplier.Status, 
  dbo.Supplier.AddressCode, 
  dbo.Supplier.Address,
  dbo.Supplier.HouseNo,
  dbo.Supplier.Street,
  dbo.Supplier.City, 
  dbo.Supplier.Country,
  dbo.Supplier.ZipCode,
  dbo.Supplier.StartDate,
  dbo.Supplier.CreditLimit, 
  dbo.Supplier.FinancialGroup,
  dbo.Supplier.LastTransactionDate, 
  dbo.Fact_PurchaseOrder.Company, 
  ISNULL(dbo.Fact_PurchaseOrder.[Purchase Order], dbo.Fact_AccountPayables.[PO Number]) AS PurchaseOrder,
  ISNULL(dbo.Fact_PurchaseOrder.Sequence, dbo.Fact_AccountPayables.Line) AS POSequence,
  dbo.Fact_PurchaseOrder.[Order Quantity], 
  dbo.Fact_PurchaseOrder.[Per Purchase Unit], 
  dbo.Fact_PurchaseOrder.[Per Quantity Price],
  dbo.Fact_PurchaseOrder.[Purchase price unit],
  dbo.Fact_PurchaseOrder.[Total Order Amount],
  dbo.Fact_PurchaseOrder.Currency, 
  dbo.Fact_PurchaseOrder.[Rate Date], 
  dbo.Fact_PurchaseOrder.[Actual Receipt Date],
  dbo.Fact_PurchaseOrder.[Receipt No],
  dbo.Fact_PurchaseOrder.[Receipt Sequence], 
  dbo.Fact_PurchaseOrder.[Received Quantity],
  dbo.Fact_PurchaseOrder.[Approved Quantity], 
  dbo.Fact_PurchaseOrder.[Purchase Office], 
  dbo.Fact_PurchaseOrder.[Invoice Number], 
  dbo.Fact_PurchaseOrder.[Invoice Date], 
  dbo.Fact_PurchaseOrder.[Invoice Quantity],
  dbo.Fact_PurchaseOrder.[Invoice Amount],
  dbo.Fact_AccountPayables.InvoiceNumber, 
  dbo.Fact_AccountPayables.Type AS InvoiceType, 
  dbo.Fact_AccountPayables.[Order Type] AS OrderInvoiceType,
  dbo.Fact_AccountPayables.AP_Balance_EUR, 
  dbo.Fact_AccountPayables.[Invoice Amount_EUR],
  dbo.Fact_AccountPayables.supplier_key AS EXPR2,
  dbo.Fact_AccountPayables.[IntercompanyTrade Order No] AS EXPR23, 
  dbo.Fact_AccountPayables.[IntercompanyTrade Line Number] AS EXPR24,
  dbo.Fact_AccountPayables.[Intercompany Trade Financial Company] AS EXPR25, 
  dbo.Fact_AccountPayables.[Intercompany Trade Purchase Company] AS EXPR26,
  dbo.Fact_AccountPayables.InvoiceNumber,
  dbo.Fact_AccountPayables.DueDate,
  dbo.Fact_AccountPayables.DocDate, 
  dbo.Fact_PurchaseOrder.[Order Date],
  dbo.Fact_AccountPayables.[Invoice Amount_EUR],
  CASE
    WHEN dbo.Fact_PurchaseOrder.[Receipt No] = ' ' THEN dbo.Fact_PurchaseOrder.[Total Order Amount]
    WHEN dbo.Fact_AccountPayables.InvoiceNumber IS NULL THEN dbo.Fact_PurchaseOrder.[Total Order Amount]
  END AS ORDERBALANCE,
  dbo.Supplier.CreditLimit - (ORDERBALANCE + dbo.Fact_AccountPayables.[Invoice Amount_EUR]) AS Availablecredit
FROM
  dbo.Supplier 
  LEFT JOIN dbo.Fact_PurchaseOrder ON dbo.Supplier.supplier_key = dbo.Fact_PurchaseOrder.buyfrom_supplier_key
  LEFT JOIN dbo.Fact_AccountPayables ON
    dbo.Fact_AccountPayables.supplier_key = dbo.Supplier.supplier_key AND
    dbo.Fact_AccountPayables.[PO Number] = dbo.Fact_PurchaseOrder.[Purchase Order]   AND 
    dbo.Fact_AccountPayables.[PO Line] = dbo.Fact_PurchaseOrder.Sequence

如果没有供应商，则在事实采购订单中筛选某些行。您需要对所有表进行完全外部联接，以获取所有应付款行的数据，而不管它们是否有供应商。

FROM  dbo.Supplier 
FULL OUTER JOIN dbo.Fact_PurchaseOrder ON dbo.Supplier.supplier_key = dbo.Fact_PurchaseOrder.buyfrom_supplier_key
FULL OUTER JOIN dbo.Fact_AccountPayables ON dbo.Fact_AccountPayables.supplier_key = dbo.Supplier.supplier_key AND
                                                        dbo.Fact_AccountPayables.[PO Number] = dbo.Fact_PurchaseOrder.[Purchase Order]   AND 
                                                    dbo.Fact_AccountPayables.[PO Line] = dbo.Fact_PurchaseOrder.Sequence