Question

我的数据库postgresql ver上发生了一场小小的灾难。 8.4。请帮帮我！

我必须在我的网页上关注我的访问者，如：

表历史记录

whereseenlast   varchar(50)
uniqid          varchar(13) primary key
whenseenlast    timestamp
bla1            varchar(10)
bla2            varchar(10)
bla3            varchar(10)

示例：

where1.html, **1**, 2000-01-01 00:00:00, somebla1, somebla2, somebla3
where2.html, **1**, 2005-05-05 05:06:07, somebla1, somebla2, somebla3
...
where12345.html, **1**, 2012-11-22 11:22:33, somebla1, somebla2, somebla3

表访问者（uniqid是主键，每个访问者只能在此表中找到一次）

wherseenfirst   varchar(50)
whereseenlast   varchar(50)
whenseenfirst   timestamp
whenseenlast    timestamp
uniqid          varchar(13) PRIMARY KEY
notes           varchar (20)
xyz             varchar(20)

示例：

where1.html, where12345.html, 2000-01-01 00:00:00,  2012-11-22 11:22:33, 1, somenote, somexyz

我不小心删除了TABLE访客，但我没有备份。但是，我确信专家很容易通过查询从TABLE历史记录重建TABLE访问者。

（我知道我无法从TABLE 历史记录恢复注释和 xyz COLUMNS，但这不是什么大不了的事）

你能建议解决方案吗？

Answer 1

这是数据库类的归属分配吗？

首先，您必须为每位访问者选择历史记录中最新和最早的行：

select uniqid, max(whenseenlast) from history group by uniqid

和

select uniqid, min(whenseenlast) from history group by uniqid

然后选择与历史记录表匹配的记录，如下所示：

select frst.uniqid, whenseenfirst, whereseenfirst, whenseenlast, whereseenlast
from
(select h.uniqid, mn.mnseen as whenseenfirst, h.whereseenlast as whereseenfirst
    from history h
    join (select uniqid, min(whenseenlast) as mnseen from history group by uniqid) mn
    on mn.mnseen = h.whenseenlast and mn.uniqid = h.uniqid) frst,
(select h.uniqid, mx.mxseen as whenseenlast, h.whereseenlast as whereseenlast
    from history h
    join (select uniqid, max(whenseenlast) as mxseen from history group by uniqid) mx
    on mx.mxseen = h.whenseenlast and mx.uniqid = h.uniqid) lst
where frst.uniqid = lst.uniqid

您可以在SQLFiddle

上进行验证

Answer 2

尝试：

SELECT DISTINCT ON (uniqid) *
FROM history
ORDER BY uniqid, whenseenlast DESC

它将为您提供每个用户的最新历史记录（每DISTINCT uniqid个）。

SELECT DISTINCT ON (uniqid) *
FROM history
ORDER BY uniqid, whenseenlast

获取第一条历史记录。

INSERT INTO visitors (uniqid, whereseenlast, whenseenlast, whenseenfirst, whenseenlast ...)
SELECT seen_last.uniqid, seen_last.whereseenlast, seen_last.whenseenlast, seen_first.whereseenlast, seen_first.whenseenlast ...
FROM 
     (SELECT DISTINCT ON (uniqid) *
      FROM history
      ORDER BY uniqid, whenseenlast DESC) seen_last
JOIN
     (SELECT DISTINCT ON (uniqid) *
      FROM history
      ORDER BY uniqid, whenseenlast) seen_first
  ON seen_last.uniqid = seen_first.uniqid

它会重建你的桌子。

详细信息：DISTINCT ON

Answer 3

选择 h1.uniqid， h1.whereseenlast asseenfirst， h1.whenseenlast asseefirst， h3.whereseenlast， h3.whenseenlast 来自历史h1 离开加入历史h2 在h1.uniqid = h2.uniqid 和h1.whenseenlast＆lt; h2.whenseenlast

加入历史记录h3 在h1.uniqid = h3.uniqid 离开加入历史h4 在h3.uniqid = h4.uniqid 和h3.whenseenlast＆gt; h4.whenseenlast

，其中 h2.uniqid为null 和h4.uniqid为空

使用查询从TABLE历史记录重建TABLE访问者？

3 个答案: