T-SQL-客户链接

使用 case 带的表达式 first_value 可以为您提供所需的结果:

SELECT CustomerID, CustomerName, CustomerTaxNumber, CreatedDate,
        CASE WHEN  CustomerTaxNumber IS NULL THEN
            FIRST_VALUE(CustomerID) OVER(PARTITION BY CustomerName ORDER BY CreatedDate)
        ELSE
            FIRST_VALUE(CustomerID) OVER(PARTITION BY CustomerTaxNumber ORDER BY CreatedDate)
        END As ParentCompanyID

FROM #Temp

CREATE TABLE #Temp
(
  CustomerID        INT,
  CustomerName      VARCHAR(20),
  CustomerTaxNumber INT,
  CreatedDate       DATE
)

INSERT INTO #Temp
VALUES (8, 'Company PTY',1234, '2019-09-20'),
       (2, 'Company PT', 1234, '2019-09-24'),
       (3, 'Company PTY',NULL, '2019-09-29')

SELECT DS.[CreatedDate] AS [FirstEntry]
      ,DS.[CustomerID] AS [ParentCompanyID]
      ,#Temp.*
FROM #Temp
CROSS APPLY
(
    SELECT TOP 1 *
    FROM #Temp
    ORDER BY CreatedDate
) DS


DROP TABLE #Temp

你是说条件很简单-得到第一条记录。如果需要以某种方式对记录进行分组,可以在 CROSS APPLY 条款。