Question

这是一个我一直绞尽脑汁的问题。假设我有一个表，其中包含一系列时间戳和一个部件号作为主键。该表存储增量更改，这意味着对于每个时间戳，如果字段更改，则记录该更改。如果该字段未更改，则对于新时间戳，它为NULL。这是基本的想法。

 part | timestamp | x-pos | y-pos | status
------+-----------+-------+-------+--------
 a5   |       151 |     5 |    15 |      g
 a5   |       153 |  NULL |    17 |   NULL

(part, timestamp)是主键。第二条记录中的NULL表示自第一条记录以来未更改的值。

我希望能够选择按部件分组的每个字段的最新值。例如，给定上述条目，对于部分a5，结果将为153,5,17，g。

到目前为止，我已将此黑客一起查询。

    ((SELECT x-pos FROM part_changes WHERE x-pos IS NOT NULL
    ORDER BY timestamp DESC
    LIMIT 1)

    UNION

    (SELECT y-pos FROM part_changesWHERE y-pos IS NOT NULL
    ORDER BY timestamp DESC
    LIMIT 1)

    UNION

    (SELECT status FROM part_changes WHERE status IS NOT NULL
    ORDER BY timestamp DESC
    LIMIT 1))

但这会返回一个列，这意味着我可以使用group-by进行组织。

必须有更优雅的做事方式，例如以创造性的方式使用COALESCE或IS NULL。但是我被困住了，无法弄明白。有人有个主意吗？

不，我无法改变数据库结构。

编辑：鲁赫克有正确的想法。现在唯一的问题是按部分分组。我似乎无法通过多个部分绕过LIMIT 1进行分组。有任何想法吗？

mdahlman，我对postgresql中的分析函数不太熟悉。因此，如果该解决方案比复杂查询更容易，那么请务必发布您的想法。

编辑2：谢谢大家的帮助。我想我已经掌握了我需要做的事情。

Answer 1

而不是使用UNION，听起来你真的想要字段列表中的子查询。也就是说，您需要(SELECT ...) UNION (SELECT ...) UNION (SELECT ...)而不是SELECT (SELECT ...), (SELECT ...), (SELECT ...)。

例如：

SELECT part,
       ( SELECT x_pos
           FROM part_changes
          WHERE part = pc.part
            AND x_pos IS NOT NULL
          ORDER
             BY timestamp DESC
          LIMIT 1
       ) AS x_pos,
       ( SELECT y_pos
           FROM part_changes
          WHERE part = pc.part
            AND y_pos IS NOT NULL
          ORDER
             BY timestamp DESC
          LIMIT 1
       ) AS y_pos,
       ( SELECT status
           FROM part_changes
          WHERE part = pc.part
            AND status IS NOT NULL
          ORDER
             BY timestamp DESC
          LIMIT 1
       ) AS status
  FROM ( SELECT DISTINCT
                part
           FROM part_changes
       ) AS pc
;

但是在这一点上我真的会考虑编写一个存储过程。

可替换地：

SELECT DISTINCT
       part,
       FIRST_VALUE(x_pos) OVER
         ( PARTITION BY part
               ORDER BY CASE WHEN x_pos IS NULL
                             THEN NULL
                             ELSE TIMESTAMP
                         END DESC NULLS LAST
         ) AS x_pos,
       FIRST_VALUE(y_pos) OVER
         ( PARTITION BY part
               ORDER BY CASE WHEN y_pos IS NULL
                             THEN NULL
                             ELSE TIMESTAMP
                         END DESC NULLS LAST
         ) AS y_pos,
       FIRST_VALUE(status) OVER
         ( PARTITION BY part
               ORDER BY CASE WHEN status IS NULL
                             THEN NULL
                             ELSE TIMESTAMP
                         END DESC NULLS LAST
         ) AS status
  FROM part_changes
;

Answer 2

只有一个部分，这应该给你答案..感谢 ruakh

但我不喜欢这个版本..

SELECT 
    (SELECT timestamp  FROM part_changes WHERE part = $part 
    ORDER BY timestamp DESC
    LIMIT 1) as timestamp,

    (SELECT x-pos FROM part_changes WHERE part = $part and x-pos IS NOT NULL
    ORDER BY timestamp DESC
    LIMIT 1) as xpos,

    (SELECT y-pos FROM part_changes WHERE part = $part and  y-pos IS NOT NULL
    ORDER BY timestamp DESC
    LIMIT 1) as ypos,

    (SELECT status FROM part_changes WHERE part = $part and status IS NOT NULL
    ORDER BY timestamp DESC
    LIMIT 1)) as status

Answer 3

数据库：选择最后的非空条目

4 个答案: