Question

我有4个表需要从中提取数据。我需要计算为单个事件签名的人数，并查看用户是否申请参加活动。

这些是我的表格设置：

TABLE: users
+----+----------+-------+--------+-------+
| id | username | level | class  | guild |
+----+----------+-------+--------+-------+
| 1  | example1 | 100   | Hunter | blah  |
| 2  | example2 | 105   | Mage   | blah2 |
| 3  | example3 | 102   | Healer | blah  |
+----+----------+-------+--------+-------+
ID is primary

TABLE: event_randoms
+----+----------+-------+--------+----------+----------+
| id | username | level | class  | apped_by | event_id |
+----+----------+-------+--------+----------+----------+
| 1  | random1  |  153  | Hunter |    3     |    3     |
| 2  | random2  |  158  | Healer |    3     |    1     |
| 3  | random3  |  167  | Warrior|    1     |    3     |
+----+----------+-------+--------+----------+----------+
ID is primary
apped_by should be foreign key to users.id
event_id should be foreign key to events.id

TABLE: events
+----+------------+------------+-----------+-----------+-----------+
| id | event_name | event_date | initiator | min_level | max_level |
+----+------------+------------+-----------+-----------+-----------+
| 1  |   event1   |    date1   |     1     |    100    |     120   |
| 2  |   event2   |    date2   |     1     |    121    |     135   |
| 3  |   event3   |    date3   |     1     |    100    |     120   |
| 4  |   event4   |    date4   |     1     |    150    |     200   |
+----+------------+------------+-----------+-----------+-----------+
ID is primary

TABLE: event_apps
+----+----------+--------------+
| id | event_id | applicant_id |
+----+----------+--------------+
| 1  |    3     |      2       |
| 2  |    4     |      2       |
| 3  |    3     |      1       |
| 4  |    1     |      3       |
+----+----------+--------------+
ID is primary
event_id should be foreign key to events.id
applicant_id should be foreign key to users.id

我将是第一个承认我对此非常陌生的人。我刚刚学会了如何使用MySQL。我可以从单个表中获取内容，但我不确定如何从多个表中获取。

这是我试过的SQL查询

SELECT DD_events.id, event_id, applicant_id, guild, level, class, DD_users.id
      FROM DD_events, DD_event_apps, DD_users
      WHERE DD_event_apps.event_id = DD_events.id
      AND DD_event_apps.applicant_id = DD_users.id

并尝试print_r数组，但数组变空。

所以有几个问题与此有关： 1：如何计算和显示有多少人（用户和randoms）注册参加活动？例如：事件3应该总共有4个（2个用户和2个randoms）

2：如何查看特定个人是否为某个活动签名并根据他们是否显示文本？例如：用户1已注册参加活动3，因此它将被注册＆＃34;但是未签名的用户2将显示＆＃34;未注册＆＃34;

3：我想显示2个表中为特定事件签名的人的信息，1表示用户，另一个表示randoms。例如：事件3在users表下将有2个用户信息（用户名，公会，类，级别），然后在随机表中有2个随机用户信息（名称，类，级别，用户应用此人）。

即使您可以回答1个部分，也可以感谢任何帮助。

Answer 1

我认为这将是您的基本查询：

SELECT
    event.id,
    app.applicant_id,
    usr.guild,
    usr.level,
    usr.class,
    usr.id AS Userid
FROM
    DD_events event
JOIN
    DD_event_apps app
    ON  (event.id = app.event_id)
LEFT JOIN
    DD_users usr
    ON  (app.user_id = usr.id)

您可以对此进行修改以进行聚合，如下所示：

SELECT
    event.id,
    COUNT(app.applicant_id) AS ApplicantCount,
    COUNT(DISTINCT usr.guild) AS UniqueGuilds,
    COUNT(DISTINCT usr.level) AS UniqueLevels,
    COUNT(DISTINCT usr.class) AS UniqueClasses,
    COUNT(DISTINCT usr.id) AS UniqueUsers
FROM
    DD_events event
JOIN
    DD_event_apps app
    ON  (event.id = app.event_id)
LEFT JOIN
    DD_users usr
    ON  (app.user_id = usr.id)
GROUP BY
    event.id

我可以为你编写这些脚本，但我认为这为你提供了一个很好的起点。当你试图获得你想要的结果时，你会发现T-SQL相当简单。希望这有帮助！

Answer 2

<?php $query = "SELECT count(*) AS numbuh FROM DD_event_apps WHERE event_id = {$row['id']}";
try
{
    // These two statements run the query against your database table.
    $stmt = $db->prepare($query);
    $stmt->execute();
}
catch(PDOException $ex)
{
    // Note: On a production website, you should not output $ex->getMessage().
    // It may provide an attacker with helpful information about your code. 
    die("Failed to run query: " . $ex->getMessage());
}
echo($query);
// Finally, we can retrieve all of the found rows into an array using fetchAll
$count = $stmt->fetchAll();
echo($count['numbuh']); ?>

选择多个表MySQL并检索不同的数据

2 个答案: