数据库:选择最后的非空条目

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.NULL第二条记录中的s表示自第一条记录以来未改变的值.

我希望能够做的是为部件分组的每个字段选择最新值.例如,给定上述条目,对于a5部分,结果将为153,5,17,g.

截至目前,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:鲁赫有正确的想法.现在唯一的问题是按部分分组.我似乎无法绕过LIMIT 1多个部分进行分组.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,那么通过各种方式发布您的想法.

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a UNION,听起来你真的想要字段列表中的子查询.也就是说,而不是(SELECT ...) UNION (SELECT ...) UNION (SELECT ...)你想要的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
;

但在这一点上,我真的会考虑编写一个存储过程.

或者:

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
;