sql - 用值替换 NULL

Question

这是我的挑战: 我有一个日志表，每次更改记录时都会添加一条新记录，但为每个记录中的每个未更改值放置一个 NULL 值。换句话说，仅设置更改的值，每行中其余未更改的字段仅具有 NULL 值。 现在我想将每个 NULL 值替换为其上面的非 NULL 值，如下所示:

源表:Task_log

ID  Owner       Status      Flag
1   Bob         Registrar   T
2   Sue         NULL        NULL
3   NULL        NULL        F
4   Frank       Admission   T
5   NULL        NULL        F
6   NULL        NULL        T

期望输出表:Task_log

ID  Owner       Status      Flag
1   Bob         Registrar   T
2   Sue         Registrar   T
3   Sue         Registrar   F
4   Frank       Admission   T
5   Frank       Admission   F
6   Frank       Admission   T

如何编写一个查询来生成所需的输出表？

Answer 1

SQLServer 2012新的窗口函数是FIRST_VALUE，它的名字很直接，可以通过OVER子句进行分区，使用前需要先划分数据 block 中的每一列，当找到一个值时，一个列的 block 开始。

With Block As (
  Select ID
       , Owner
       , OBlockID = SUM(Case When Owner Is Null Then 0 Else 1 End) 
                    OVER (ORDER BY ID)
       , Status
       , SBlockID = SUM(Case When Status Is Null Then 0 Else 1 End) 
                    OVER (ORDER BY ID)
       , Flag
       , FBlockID = SUM(Case When Flag Is Null Then 0 Else 1 End) 
                    OVER (ORDER BY ID)
  From   Task_log
)
Select ID
     , Owner = FIRST_VALUE(Owner) OVER (PARTITION BY OBlockID ORDER BY ID)
     , Status = FIRST_VALUE(Status) OVER (PARTITION BY SBlockID ORDER BY ID)
     , Flag = FIRST_VALUE(Flag) OVER (PARTITION BY FBlockID ORDER BY ID)
FROM   Block

SQLFiddle演示

UPDATE 查询很容易派生