SQL语句中的第一条记录

Question

为此，我需要链接和其他表以按工厂过滤，从而为每个条目创建多个记录。 我希望得到一个的第一个条目。

SELECT 
    OrderHead.CreditOverride, 
    OrderHead.OpenOrder, Customer.CreditHold, 
    OrderHead.OrderNum, Customer.Name, 
    OrderHead.EntryPerson, OrderHead.OrderDate, 
    Customer.TermsCode, OrderHead.ShipToCustNum, OrderRel.Plant
FROM  
    Customer 
INNER JOIN
    OrderHead ON Customer.Company = OrderHead.Company 
             AND Customer.CustNum = OrderHead.BTCustNum 
INNER JOIN
    OrderRel ON OrderHead.OrderNum = OrderRel.OrderNum
WHERE 
    (OrderHead.CreditOverride = 0) 
    AND (OrderHead.OpenOrder = 1)  
    AND (Customer.CreditHold = 1) 
    AND (OrderRel.Plant = 'mfgsys')

试图从订单头中获取第一个唯一记录。

Answer 1

使用窗口功能。 您可以完全在FROM子句中完成此操作：

FROM Customer c INNER JOIN
     (SELECT oh.*,
             ROW_NUMBER() OVER (PARTITION BY Company, BTCustNum ORDER BY OrderDate ASC) as seqnum
      FROM OrderHead oh
     ) oh
     ON c.Company = oh.Company AND
        c.CustNum = oh.BTCustNum AND
        oh.seqnum = 1 INNER JOIN
     OrderRel orr
     ON oh.OrderNum = orr.OrderNum

请注意，我用更简单的表别名替换了表名，您应该在查询的其余部分中重复该表别名。