如何转换查询以使用联接？

如何解决如何转换查询以使用联接？

我想编写一个查询，该查询使用“联接概念”来为每个订单返回自同一位客户的先前订单以来经过的天数。

我正在使用AdventureWorks2017数据库的表Sales.SalesOrderHeader

我尝试了以下代码，但实际上我希望通过联接概念来实现它：

select customerid,orderdate,salesorderid,datediff(day,(select top(1) s1.orderdate 
             from  sales.SalesOrderHeader s1
             where s1.CustomerID=s.CustomerID
                   and (s1.OrderDate = s.Orderdate 
                       and s1.SalesOrderID < s.Salesorderid
                       or  s1.OrderDate < s.orderdate)
             order by s1.orderdate desc,s1.SalesOrderID desc
             ),orderdate) as Daysdifference  
    from sales.SalesOrderHeader s  
    order by CustomerID,OrderDate,SalesOrderID

解决方法

从现有查询开始，一种简单的方法是使用横向联接：

select s.customerid,s.orderdate,s.salesorderid,datediff(day,s1.orderdate) days_difference
from sales.salesorderheader s
cross apply (
    select top(1) s1.orderdate 
    from sales.salesorderheader s1
    where s1.customerid = s.customerid
    and (s1.orderdate < s.orderdate) or (s1.orderdate = s.orderdate and s1.id < s.id)
    order by s1.orderdate desc,s1.id desc
) s1

这回答了您关于如何通过联接实现逻辑的问题。但是，我要指出的是，使用lag()可以更有效地表达这一点：

select customerid,orderdate,salesorderid,lag(orderdate) over(partition by customer_id order by order_date,id) days_difference
from sales.salesorderheader s

,

我会这样写

SELECT s.CustomerID,s.OrderDate,s.SalesorderID,s1.OrderDate,s.OrderDate) as Daysdifference  
FROM sales.SalesOrderHeader s
OUTER APPLY (
  SELECT top(1) s1.OrderDate-- maybe MAX(s1.OrderDate) ?
  FROM sales.SalesOrderHeader s1
  WHERE s1.CustomerID = s.CustomerID
    AND ((s1.OrderDate = s.OrderDate 
          AND s1.SalesOrderID < s.SalesOrderID) -- I've added parenthesis
       OR  s1.OrderDate < s.OrderDate)
  ORDER BY s1.OrderDate DESC,s1.SalesOrderID DESC
) s1
ORDER BY s.CustomerID,s.SalesOrderID

或者类似的东西？

SELECT s.customerID,s.orderDate,s.SalesOrderID,MAX(s1.OrderDate),s.OrderDate) as Daysdifference  
FROM sales.SalesOrderHeader s
LEFT JOIN sales.SalesOrderHeader s1 ON s1.CustomerID = s.CustomerID
                                AND ((s1.OrderDate = s.OrderDate 
                                  AND s1.SalesOrderID < s.SalesOrderID) -- I've added parenthesis
                                OR  s1.OrderDate < s.OrderDate))
GROUP BY s.CustomerID,s.salesorderId
ORDER BY s.CustomerID,s.SalesOrderID