Question

今天，我想获取已选择或具有最近日期的列表中所有事件的列表。我实际上使用以下代码

SELECT choices.id AS id, events.id AS event_id,
events.owner_id, choices.selected, 
choices_dates.label AS events_date
FROM events
LEFT JOIN polls ON (events.id = polls.event_id)
LEFT JOIN choices ON (polls.id = choices.poll_id)
LEFT JOIN choices_dates ON (choices.id = choices_dates.choice_id)
WHERE polls.kind = 'date'
ORDER BY events.id, choices.selected DESC, events_date DESC;

我得到了这个结果：

id         event_id         owner_id         selected         events_date
2          1         1         0         2011-04-20 12:00:00
1          1         1         0         2011-04-20 00:00:00
7          2         1         0         2011-04-20 12:00:00
6          2         1         0         2011-04-20 00:00:00
13         3         1         0         2011-04-22 15:57:54
12         3         1         0         2011-04-20 12:00:00
11         3         1         0         2011-04-20 00:00:00

但这就是我想要的：

id         event_id         owner_id         selected         events_date
2          1         1         0         2011-04-20 12:00:00
7          2         1         0         2011-04-20 12:00:00
13         3         1         0         2011-04-22 15:57:54

我无法进行嵌套查询，因为我正在创建一个VIEW，并且group by给出了随机结果（我获取了event_id 1的第一行，以及event_id 2的最后一行，...）

现在，我可以使用php函数完成这项工作，但它不是优化，而且我的代码很脏。

有人知道如何使用一个出色的SQL函数吗？

谢谢，凯文

编辑：我已经成功地做了我想做的事情：我使用了两种不同的视图：

创建VIEWview_events_maxAS 选择 events.id AS event_id， MAX（choices.selected）AS max_selected， MAX（choices_dates.label）AS max_events_date 来自事件 LEFT JOIN轮询ON（events.id = polls.event_id） LEFT JOIN选项ON（polls.id = choices.poll_id） LEFT JOIN choices_dates ON（choices.id = choices_dates.choice_id） WHERE polls.kind ='date' GROUP BY event_id

和

创建视图view_events_correct AS（选择 choices.id AS id， events.id AS event_id， events.owner_id， choices.selected， choices_dates.label AS event_date 来自事件 LEFT JOIN轮询ON（events.id = polls.event_id） LEFT JOIN选项ON（polls.id = choices.poll_id） LEFT JOIN choices_dates ON（choices.id = choices_dates.choice_id INNER JOIN view_events_max ON（events.id = view_events_max.event_id AND（choices.selected = 1 OR（choices.selected = view_events_max.max_selected AND choices_dates.label = view_events_max.max_events_date））））

谢谢大家的帮助，凯文

Answer 1

我知道在Oracle中你可以使用分析函数来做到这一点。我不知道mySql是否有类似内容...

select id, event_id, owner_id, selected, events_date from (
SELECT choices.id AS id, events.id AS event_id, events.owner_id as owner_id,
choices.selected as selected,
choices_dates.label AS events_date,
max(choices_date.label) over (partition by events.id) as max_events_date
FROM events LEFT JOIN polls ON (events.id = polls.event_id) LEFT JOIN choices ON
(polls.id = choices.poll_id) LEFT JOIN choices_dates ON (choices.id =
choices_dates.choice_id)
WHERE polls.kind = 'date') innerview
where events_date = max_events_date
order by event_id, selected DESC, events_date DESC;

Answer 2

这样的事情（未经测试，缺少模式描述和数据库软件版本）是否有效？

select choices.id as id
     , events.id as event_id
     , events.owner_id
     , choices.selected
     , choices_dates.label as events_date
from
(
    SELECT events.id as max_event_id
         , max(choices_dates.label) as max_events_date
    FROM events
    LEFT JOIN polls ON (events.id = polls.event_id)
    LEFT JOIN choices ON (polls.id = choices.poll_id)
    LEFT JOIN choices_dates ON (choices.id = choices_dates.choice_id)
    WHERE polls.kind = 'date'
    GROUP BY events.id
) 
LEFT JOIN polls ON (events.id = polls.event_id)
LEFT JOIN choices ON (polls.id = choices.poll_id)
LEFT JOIN choices_dates ON (choices.id = choices_dates.choice_id)
where events_date = max_events_date
  and events.id = max_event_id

获取每个ID的第一行

2 个答案: