sql - 是否可以从一对多关系中的每一列获取最新值？

Question

使用此数据集:

| RECID |  ID  | VALUE1 | VALUE2 | VALUE3 |                          RECDT |
----------------------------------------------------------------------------
|     1 | 1-01 |      1 |      2 |      3 | January, 01 2013 00:00:00+0000 |
|     2 | 1-01 |      3 | (null) | (null) | January, 02 2013 00:00:00+0000 |
|     3 | 1-01 | (null) | (null) |      1 | January, 03 2013 00:00:00+0000 |

是否可以通过简单的查询返回以下结果？

|  ID  | VALUE1 | VALUE2 | VALUE3 |
-----------------------------------
| 1-01 |      3 |      2 |      1 | 

数据集需要根据日期和 PtID 返回每列的最新值。因此，举例来说，如果我对 2013 年 1 月 2 日之前 PTID 的所有更改感兴趣，结果将如下所示:

|  ID  | VALUE1 | VALUE2 | VALUE3 |
-----------------------------------
| 1-01 |      3 |      2 |      3 |

架构已于 sqlfiddle 启动对于任何有兴趣的人。

Answer 1

下面是一种适用于多种 SQL 方言的方法:

select ptid,
       (select value1 from t t2 where t2.ptid = t.ptid and value1 is not null order by recdt desc limit 1
       ) as Value1,
       (select value2 from t t2 where t2.ptid = t.ptid and value2 is not null order by recdt desc limit 1
       ) as Value2,
       (select value3 from t t2 where t2.ptid = t.ptid and value3 is not null order by recdt desc limit 1
       ) as Value3
from t
where ptid = '1-01'
group by ptid

某些数据库可能更喜欢 top 或 recnum = 1 而不是 limit。

在MySQL中，你还可以这样做:

select ptid,
       substring_index(group_concat(value1 order by recdt desc), ',', 1) as Value1,
       substring_index(group_concat(value2 order by recdt desc), ',', 1) as Value2,
       substring_index(group_concat(value3 order by recdt desc), ',', 1) as Value3
from t
group by ptid

这会产生将值转换为字符串的副作用。您可以转换回您想要的类型。另外，如果值可能包含逗号，那么您可能需要使用不同的分隔符。