这是一个有点冗长的问题，对此我深表歉意。我的 SQL 技能严重缺乏（我想尽快纠正）。结果，我真的无法理解如何解决我遇到的这个问题。
基本上，我们的项目是将用户通知存储在一个表中notifications。表结构如下所示：
+-----------+-----------+----------------+------------+
| user_id | subject | action | date |
+-----------+-----------+----------------+------------+
| 1 | 2 | started_follow | 1371034287 |
| 1 | 2 | stopped_follow | 1371034287 |
| 2 | 5 | added_item | 1371034287 |
+-----------+-----------+----------------+------------+
user_id始终包含执行该操作的用户的 ID，并且date显然是注册通知的日期。棘手的部分是subject对另一个表的 ID 的引用，并且该表在很大程度上取决于action列的值。
所以例如，在样本数据的前两条记录中，subject是对表中的一个ID的引用users（即被关注的用户，然后被取消关注的用户）。在第三条记录中，subject是对items表中 ID 的引用。
我们还需要执行几个JOIN语句，具体取决于action. 因此，如果是added_item例如，我们需要JOIN几个其他表（以检查设置和其他要求）。
我在代码中遇到了一个遗留函数，它本质上检查表中自指定日期以来给定用户的通知数量。之前的开发者只是简单的使用了一系列的查询，然后返回了几个语句的总数SELECT COUNT(*)如下（请注意，这都是在PHPUser类里面）：
// Get the number of notifications since the specified time (or of all time):
public function countNotifications($since = '')
{
$sinceString = ($since == '') ? '' : "AND `date` > '$since'";
// Notifications when someone follows $this:
$started_following = $this->_database->query("SELECT COUNT(*) AS `count`
FROM `notifications`
WHERE `action` = 'started_following'
AND `subject` = '{$this->id}'
$sinceString
AND `notifications`.`user_id` != '{$this->id}'")->fetchObject();
// Notifications when someone stops following $this:
$stopped_following = $this->_database->query("SELECT COUNT(*) AS `count`
FROM `notifications`
WHERE `action` = 'stopped_following'
AND `subject` = '{$this->id}'
$sinceString
AND `notifications`.`user_id` != '{$this->id}'")->fetchObject();
// Notifications when someone sends $this a message:
$sent_message = $this->_database->query("SELECT COUNT(*) AS `count`
FROM `notifications`
WHERE `action` = 'sent_message'
AND `subject` = '{$this->id}'
$sinceString
AND `notifications`.`user_id` != '{$this->id}'")->fetchObject();
// Notifications when someone favorites $this' items:
$favorited_item = $this->_database->query("SELECT COUNT(*) AS `count`
FROM `notifications`
INNER JOIN `items` ON `notifications`.`subject` = `items`.`id`
INNER JOIN `categories` ON `items`.`category` = `categories`.`id`
WHERE `notifications`.`action` = 'favorited_item'
AND `categories`.`owner` = '{$this->id}'
$sinceString
AND `notifications`.`user_id` != '{$this->id}'")->fetchObject();
// Notifications when someone adds a comment to $this' items:
$comments = $this->_database->query("SELECT COUNT(*) AS `count`
FROM `notifications`
INNER JOIN `items` ON `notifications`.`subject` = `items`.`id`
INNER JOIN `categories` ON `items`.`category` = `categories`.`id`
WHERE `notifications`.`action` = 'added_comment'
AND `categories`.`owner` = '{$this->id}'
$sinceString
AND `notifications`.`user_id` != '{$this->id}'")->fetchObject();
// Notifications when a follower of $this adds a new item:
$new_items = $this->_database->query("SELECT COUNT(*) AS `total`
FROM `notifications`
INNER JOIN `categories` ON `notifications`.`subject` = `categories`.`id`
INNER JOIN (SELECT `followee` FROM `user_followers` WHERE `follower` = '{$this->id}') `followers`
WHERE `notifications`.`action` = 'added_item'
AND `followee` = `user_id`
$sinceString
AND `user_id` != '{$this->id}'")->fetchObject();
// Notifications when a follower of $this adds a new collection:
$new_collections = $this->_database->query("SELECT COUNT(*) AS `total`
FROM `notifications`
INNER JOIN `categories` ON `notifications`.`subject` = `categories`.`id`
INNER JOIN (SELECT `followee` FROM `user_followers` WHERE `follower` = '{$this->id}') `followers`
WHERE `notifications`.`action` = 'added-collection'
AND `followee` = `user_id`
$sinceString
AND `user_id` != '{$this->id}'")->fetchObject();
// Notifications when a follower of $this adds a new category:
$new_categories = $this->_database->query("SELECT COUNT(*) AS `total`
FROM `notifications`
INNER JOIN `categories` ON `notifications`.`subject` = `categories`.`id`
INNER JOIN (SELECT `followee` FROM `user_followers` WHERE `follower` = '{$this->id}') `followers`
WHERE `notifications`.`action` = 'added-category'
AND `followee` = `user_id`
$sinceString
AND `user_id` != '{$this->id}'")->fetchObject();
// Selling Notifications:
// Notifications when someone makes an offer for an item $this is selling:
$offers = $this->_database->query("SELECT COUNT(*) AS `count`
FROM `notifications`
INNER JOIN `items` ON `notifications`.`subject` = `items`.`id`
INNER JOIN `categories` ON `items`.`category` = `categories`.`id`
WHERE `notifications`.`action` = 'made_offer'
AND `categories`.`owner` = '{$this->id}'
$sinceString
AND `notifications`.`user_id` != '{$this->id}'")->fetchObject();
// Notifications when someone purchases an item $this is selling:
$purchases = $this->_database->query("SELECT COUNT(*) AS `count`
FROM `notifications`
INNER JOIN `items` ON `notifications`.`subject` = `items`.`id`
INNER JOIN `categories` ON `items`.`category` = `categories`.`id`
INNER JOIN (SELECT COUNT(*) AS `count`, `item_id`
FROM `user_favorite_items`
WHERE `user_id` = '{$this->id}') `following` ON `items`.`id` = `following`.`item_id`
WHERE `notifications`.`action` = 'bought_item'
AND `following`.`count` = 1
AND `categories`.`owner` = '{$this->id}'
$sinceString
AND `notifications`.`user_id` != '{$this->id}'")->fetchObject();
// Notifications when an item that $this favorited is listed for sale:
$item_sales = $this->_database->query("SELECT COUNT(*) AS `count`
FROM `notifications`
INNER JOIN `items` ON `notifications`.`subject` = `items`.`id`
INNER JOIN (SELECT COUNT(*) AS `count`, `item_id`
FROM `user_favorite_items`
WHERE `user_id` = '{$this->id}'
) `following` ON `items`.`id` = `following`.`item_id`
WHERE `notifications`.`action` = 'selling_item'
AND `following`.`count` = 1
$sinceString
AND `notifications`.`user_id` != '{$this->id}'")->fetchObject();
// Return the counts:
return ($started_following->count +
$stopped_following->count +
$sent_message->count +
$favorited_item->count +
$comments->count +
$new_items->count +
$new_collections->count +
$new_categories->count +
$offers->count +
$purchases->count +
$item_sales->count);
}
虽然这很好地完成了这项工作，但它使得获取指定日期的所有记录或与特定用户 ID 相关的所有记录变得极其困难。
我想我的问题真的是结合提供的众多 SQL 语句的最佳方法是什么？我已经尝试过LEFT JOIN，但正如您所见，我们需要将表连接到不同的列，具体取决于 的值notifications。action. 虽然我可以使用表别名来做到这一点，但它确实往往会返回大量冗余数据。
本质上，我想结合COUNT(*)上面给出的查询，以便我们可以简单地返回notifications.*给定用户 ID 和/或时间段的所有查询。
如果可能的话，我也想避免使用UNION（出于显而易见的原因）。
很抱歉这个冗长的问题，但我试图让一切尽可能清楚。在有人问之前，我无法更改数据结构或数据库架构，因为这是针对现有站点的。
我整理了一个SQLFiddle以使事情更加清晰。