如何比较同一个SQL Server表中的记录

Question

我的要求是，以行的各列其前一行进行比较。

1. 比较行的2行1
2. 比较行3列2

而且，如果没有区别，我需要让该列NULL。例如：第3行的request_status_id与第2行的相同，所以我需要将第3行的request_status_id更新为NULL。

有没有一个干净的方法来做到这一点？

Answer 1

您可以使用下面的UPDATE声明，它采用可LAG窗函数从SQL Server 2012起：

UPDATE #mytable 
SET request_status_id = NULL 
FROM #mytable AS m 
INNER JOIN (
    SELECT payment_history_id, request_status_id, 
    LAG(request_status_id) OVER(ORDER BY payment_history_id) AS prevRequest_status_id 
    FROM #mytable) t 
ON m.payment_history_id = t.payment_history_id 
WHERE t.request_status_id = t.prevRequest_status_id 

SQL Fiddle Demo here

编辑：

看来OP的要求是SET每列的表 至NULL，前一个值与当前值相同。在这种情况下，查询变得更加冗长。这是一个有两列设置的例子。它可以很容易地扩展到包含表的任何其他列：

UPDATE #mytable 
SET request_status_id = CASE WHEN t.request_status_id = t.prevRequest_status_id THEN NULL 
          ELSE T.request_status_id 
         END, 
    request_entity_id = CASE WHEN t.request_entity_id = t.prevRequest_entity_id THEN NULL 
          ELSE t.request_entity_id 
         END 
FROM #mytable AS m 
INNER JOIN (
    SELECT payment_history_id, request_status_id, request_entity_id, 
    LAG(request_status_id) OVER(ORDER BY payment_history_id) AS prevRequest_status_id, 
    LAG(request_entity_id) OVER(ORDER BY payment_history_id) AS prevRequest_entity_id 
    FROM #mytable) t 
ON m.payment_history_id = t.payment_history_id 

SQL Fiddle Demo here