MYSQL - 根据特定条件输出额外的列

Question

首先，我想为提供这么弱的标题道歉；我无法用更好的方式来描述它。

考虑以下情况：我们有三张表，一张用于users，一张用于records，一张用于ratings。这些表是不言自明的，但数据库的架构如下：

+---------------------+
| Tables_in_relations |
+---------------------+
| records             |
| ratings             |
| users               |
+---------------------+

records 表的架构如下：

+----------+----------------------+------+-----+---------+----------------+
| Field    | Type                 | Null | Key | Default | Extra          |
+----------+----------------------+------+-----+---------+----------------+
| id       | smallint(5) unsigned | NO   | PRI | NULL    | auto_increment |
| title    | varchar(256)         | NO   |     | NULL    |                |
| year     | int(4)               | NO   |     | NULL    |                |
+----------+----------------------+------+-----+---------+----------------+

users 表的架构如下：

+----------+----------------------+------+-----+---------+----------------+
| Field    | Type                 | Null | Key | Default | Extra          |
+----------+----------------------+------+-----+---------+----------------+
| id       | smallint(5) unsigned | NO   | PRI | NULL    | auto_increment |
| email    | varchar(256)         | NO   |     | NULL    |                |
| name     | varchar(256)         | NO   |     | NULL    |                |
| password | varchar(256)         | NO   |     | NULL    |                |
+----------+----------------------+------+-----+---------+----------------+

ratings 表显然是在其中存储评级与 record_id 和 user_id 并用作关系表。

它的架构如下：

+----------+----------------------+------+-----+---------+----------------+
| Field    | Type                 | Null | Key | Default | Extra          |
+----------+----------------------+------+-----+---------+----------------+
| id       | smallint(5) unsigned | NO   | PRI | NULL    | auto_increment |
| record_id| smallint(5) unsigned | NO   | MUL | NULL    |                |
| user_id  | smallint(5) unsigned | NO   | MUL | NULL    |                |
| rating   | int(1)               | NO   |     | NULL    |                |
+----------+----------------------+------+-----+---------+----------------+

现在，在我的应用程序中，我有一个搜索功能，可以根据某个关键字获取记录。输出还应包括某条记录的平均评分和每条记录的评分总数。这可以通过以下查询来完成：

SELECT re.id, re.title, re.year, ROUND(avg(ra.rating)) as avg_rate,
COUNT(ra.record_id) as total_times_rated
FROM records re
LEFT JOIN ratings ra ON ra.record_id = re.id
GROUP BY re.id;

这会给我以下输出：

+----+------------------------+------+----------+-------------------+
| id | title                  | year | avg_rate | total_times_rated |
+----+------------------------+------+----------+-------------------+
|  1 | Test Record 1          | 2008 |        3 |                 4 |
|  2 | Test Record 2          | 2012 |        2 |                 4 |
|  3 | Test Record 3          | 2003 |        3 |                 4 |
|  4 | Test Record 4          | 2012 |        3 |                 3 |
|  5 | Test Record 5          | 2003 |        2 |                 3 |
|  6 | Test Record 6          | 2006 |        2 |                 3 |
+----+------------------------+------+----------+-------------------+

问题：

现在，棘手的部分来了，至少对我来说是这样。在我的应用程序中，您可以搜索是否登录的记录，如果登录，我还想在上述查询中包含用户自己的评分值。

我知道我可以通过读取会话值来运行条件来检查用户是否已登录，并据此执行相应的查询。我只是不知道如何将某个用户的个人评分值包含到上述查询中。

Answer 1

您可以通过在列中添加SELECT 查询来在结果中添加用户评分：

SELECT re.id, re.title, re.year, ROUND(avg(ra.rating)) as avg_rate,
COUNT(ra.record_id) as total_times_rated,
(SELECT rating FROM ratings WHERE user_id = ? AND record_id = re.id) as user_rating
FROM records re
LEFT JOIN ratings ra ON ra.record_id = re.id
GROUP BY re.id;

我们可以从会话中获取user_id 并将其传递给此查询，以便在结果中生成user_rating 列。

假设用户可以多次评价一条记录，我使用了SUM。如果没有，我们可以将其从查询中删除。

更新

如果您不希望 GROUP BY 考虑该值，则可以将现有查询包装到另一个查询中并向其中添加一列，例如：

SELECT a.id, a.title, a.year, a.avg_rate, a.total_times_rated,
(SELECT rating FROM ratings WHERE user_id = ? AND record_id = a.id) as user_rating
FROM (SELECT re.id as id, re.title as title, re.year as year, ROUND(avg(ra.rating)) as avg_rate,
COUNT(ra.record_id) as total_times_rated
FROM records re
LEFT JOIN ratings ra ON ra.record_id = re.id
GROUP BY re.id) a;