php - 在sqlite3 : looking for SELECT query for an actual composition of members of a certain group中

Question

假设我有一个表，其中包含成员加入和离开该类型组的事件

member_id| action | group   | ts
----------------------------------------
member_1 | joins  | group_a | 1445017260
member_2 | joins  | group_b | 1445017551
member_2 | leaves | group_b | 1445017580
member_3 | joins  | group_a | 1445017860
member_4 | joins  | group_b | 1445017951
member_3 | leaves | group_a | 1445017980
member_3 | joins  | group_b | 1445017990
member_5 | joins  | group_b | 1445018051
member_6 | joins  | group_a | 1445018580
...

我想确定谁在特定时间戳的特定组中。

例如谁在时间戳 1445018680 的 group_b 中(在本例中发生所有事件之后)所需的输出是:

[member_3, member_4, member_5]

我正在使用 php 和 sqlite3。目前，我只看到有机会迭代所有现有成员并进行选择，如下所示:

$inMembers = array();
$ts    = 1445018680;
$group = "group_b";

foreach($members as $member){

  $action = $DB->Select("SELECT * FROM events_$eventid WHERE group = :group AND event_ts <= :ts ORDER BY event_ts DESC LIMIT 1", array(
    "ts"    => $ts,
    "group" => $group,
  ));

  if($action == "joins") $inMembers[] = $member;

}

DB对象是一个抽象层。我希望我的意思很清楚。我猜想有一个更优雅的解决方案。也许没有循环。查询只需要 TS。

感谢您的帮助。 疯狂

Answer 1

此查询获取每个组成员的最新时间戳:

SELECT member_id,
       MAX(event_ts)
FROM events_xxx
WHERE group = :group
  AND event_ts <= :ts
GROUP BY member_id;

要获取这些行的其他列，请将其连接回原始表:

SELECT events.member_id
FROM events_xxx
NATURAL JOIN (SELECT member_id,
                     MAX(event_ts) AS event_ts
              FROM events_xxx
              WHERE group = :group
                AND event_ts <= :ts
              GROUP BY member_id
             ) AS latest_events
WHERE events.action = 'joins';