DB2 - 有没有一种简单的方法来获得“以前的有效值”？答案

【问题标题】：DB2 - Is there a simple way to get "previous valid value"?DB2 - 有没有一种简单的方法来获得“以前的有效值”？
【发布时间】：2013-06-24 20:35:23
【问题描述】：

我不确定将问题称为“以前的有效值”是否合适。事情是这样的：

我有一个表“A”：

create table A (
  name varchar(16),
  performanceDate date,
  value int
);

name 和 PerformanceDate 都是主键。

有一个流程会每天为每个用户插入数据。因此，数据将如下所示：

select * from A;
|------+-----------------+-------|
| name | performanceDate | value |
|------+-----------------+-------|
| Joe  |      2012-05-18 | null  |
| Joe  |      2012-05-17 | 2     |
| Joe  |      2012-05-16 | null  |
| Joe  |      2012-05-15 | null  |
| Joe  |      2012-05-14 | 3     |
|------+-----------------+-------|

目前，我想获得一个 preformanceDate 介于 2012-05-16 和 2012-05-18 之间的结果集>。例如2012-05-16的值为null，2012-05-16之前的第一个有效值为2012-05-14的3。因此，结果如下：

|------+-----------------+-------|
| name | performanceDate | value |
|------+-----------------+-------|
| Joe  |      2012-05-18 |     2 |
| Joe  |      2012-05-17 |     2 |
| Joe  |      2012-05-16 |     3 |
|------+-----------------+-------|

到目前为止，我计划先将数据插入临时表（因为表“A”对我来说是只读的），然后逐个更新值。但是这种方式非常慢。你对此有什么想法吗？

【问题讨论】：

忘了提一下，该过程必须并且大多数情况下每天都为每个用户插入一条记录。如果今天没有数据，则插入“null”。
我认为这可能通过一次处理 PL 和元组来实现。因为它只需要一个排序，然后它可以使用会发出元组的历史相关进程流过元组。
我刚刚发现子查询也可以做到这一点。

标签： sql group-by db2 sql-order-by

【解决方案1】：

您可以使用 LATERAL JOIN，SQLFiddle 上还没有 DB2，下面是它在 SQL Server 中的等价物。 LATERAL 相当于 SQL Server 的 APPLY：

select x.name, x.performanceDate, coalesce(x.value, y.value) as value
from tbl x

outer apply 
( 
  -- find nearest

  select top 1 value
  from tbl
  where 

    x.value is null

    and 

    (
       name = x.name
       and value is not null 
       and performanceDate < x.performanceDate 
    )

  order by performanceDate desc

) as y
order by x.name, x.performanceDate desc

数据：

| NAME |            PERFORMANCEDATE |  VALUE |
|------|----------------------------|--------|
|  Joe | May, 18 2012 08:00:00-0700 | (null) |
|  Joe | May, 17 2012 08:00:00-0700 |      2 |
|  Joe | May, 16 2012 08:00:00-0700 | (null) |
|  Joe | May, 15 2012 08:00:00-0700 | (null) |
|  Joe | May, 14 2012 08:00:00-0700 |      3 |

输出：

| NAME |            PERFORMANCEDATE | VALUE |
|------|----------------------------|-------|
|  Joe | May, 18 2012 08:00:00-0700 |     2 |
|  Joe | May, 17 2012 08:00:00-0700 |     2 |
|  Joe | May, 16 2012 08:00:00-0700 |     3 |
|  Joe | May, 15 2012 08:00:00-0700 |     3 |
|  Joe | May, 14 2012 08:00:00-0700 |     3 |

现场测试：http://www.sqlfiddle.com/#!6/e0158/8

基于 IBM 文档..http://publib.boulder.ibm.com/infocenter/iseries/v7r1m0/index.jsp?topic=%2Fdb2%2Frbafztabref.htm

..，我希望是这样的：

select x.name, x.performanceDate, coalesce(x.value, y.value) as value
from tbl x,

lateral 
( 
  -- find nearest

  select top 1 value
  from tbl
  where 

    x.value is null

    and 

    (
       name = x.name
       and value is not null 
       and performanceDate < x.performanceDate 
    )

  order by performanceDate desc
  fetch first 1 rows only

) as y
order by x.name, x.performanceDate desc

关于 CROSS APPY/OUTER APPLY 的有趣指标：http://explainextended.com/2009/07/16/inner-join-vs-cross-apply/

另一个使用 OUTER APPLY 的例子：http://www.ienablemuch.com/2012/04/outer-apply-walkthrough.html

【讨论】：

【解决方案2】：

with tmp1(name, performanceDate, value, cid) as (
select 
   name,
   performanceDate,
   value,
   count(value) over (partition by name 
                          order by performanceDate,
                                   value nulls last) as cid from A),
tmp2(name, performanceDate, value) as (
select 
   name,
   performanceDate,
   first_value(value) over (partition by name, cid 
                                order by performanceDate, 
                                         value nulls last from tmp1)
select * from tmp2;

with tmp1 (name, performanceDate_Begin, performanceDate_End, value) as (
select 
   name,
   performanceDate as performanceDate_Begin,
   lead(performanceDate) over (partition by name 
                                   order by performanceDate,
                                            value nulls last) as performanceDate_End,value from A where value is not null),
tmp2 (name, performanceDate, value) as (
select 
   A.name,
   A.performanceDate,
   tmp1.value 
from A 
left join tmp1 on A.name = B.name 
              and A.performanceDate >= tmp1.performanceDate_Begin 
              and A.performanceDate < coalesce(tmp1.performanceDate_End,date '9999-12-31'))
select * from tmp2

【讨论】：