如何使用 codeigniter 查询获取每个产品的平均评分？

我有评级表，其中对用户给出的每个产品进行评级，我正在检索所有评级记录，但同时我想根据每个产品获得平均评级，但我无法获得输出

Query :

$this->db->select('ratings.*');
$this->db->select('select AVG(ratings) as avg_rating from ratings group by product_id');
$this->db->from('ratings');
$this->db->join('users','users.id=ratings.user_id');
$this->db->get()->result();

评级表

id   user_id product_id  rating  

1      4         1         4
 
2      5         2         4

3      6         1         2


4      7         4         4

预期输出：

id   user_id product_id  rating          avg rating

1      4         1         4               3
 
2      5         2         4               4

3      6         1         2               3


4      7         4         4               4

从表中获取数据ratings，使用左连接和 select 来求平均值。

the join() 函数 https://codeigniter.com/userguide3/database/query_builder.html?highlight=join#CI_DB_query_builder::joinCodeigniter 允许您编写选择部分而不是表名，但您需要将其放入括号中：

$this->db->select('t1.*, t2.avg_rating, t3.*');
$this->db->from('ratings t1');
$this->db->join('
                  (select product_id, avg(rating) as avg_rating 
                   from ratings 
                   group by product_id)  t2','t2.product_id=t1.product_id','left'
                );
$this->db->join('users t3','t3.id=t1.user_id','left'); 
$this->group_by('t1.userid')
$this->db->get()->result();

生成：

SELECT t1.*, t2.avg_rating, t3.*
FROM ratings t1
left join 
        (select product_id, avg(rating) as avg_rating from ratings group by product_id)  t2 
    on t2.product_id=t1.product_id
left join users t3
    on t1.user_id = t3.id
group by t1.user_id

并按照您的预期输出。