MySQL 查询 JOIN 3 个表

Question

希望加入 3 个表，但在最后一个表中遇到问题。

members

ID | name
---------
1  | John
2  | Jane
3  | Jack

member_points（成员之间可以有多个交易）

ID | date | id_from | id_to
---------------------------
1  | 8/8  |    1    |   2
2  | 8/8  |    1    |   2
3  | 8/8  |    3    |   2

member_ratings（一个会员只能评价另一个会员，一次）

ID | id_from | id_to | rating
-----------------------------
1  |    2    |   1   |   5

每个会员只能对每个会员进行一次评分，并且只能根据member_points 表对他们获得积分的会员进行评分。

我当前的查询实现了这一点，但是我在介绍第三个表时遇到了困难，该表将包含相应的评级。

这是我目前所拥有的：

$sql = '
SELECT *, 
    m.id AS id, 
    c1.id AS id_from, 
    c1.name AS name_from,
    c2.id AS id_to, 
    c2.name AS name_to
FROM member_points AS m
JOIN members AS c1 ON m.id_from = c1.id
JOIN members AS c2 ON m.id_to = c2.id
    and m.id_to='.$_SESSION["userid"].'
GROUP BY name_from';

我的目标是加入第三张桌子，这样我就可以调用相关的评级。

ID | name_from | name_to | rating
----------------------------------
1  |     2     |    1    |   5     
2  |     2     |    3    | pending

Answer 1

@Andrew 你需要像这样使用LEFT JOIN 加入member_ratings 表：

$sql = '
SELECT *, 
    m.id AS id, 
    c1.id AS id_from, 
    c1.name AS name_from,
    c2.id AS id_to, 
    c2.name AS name_to,
    IF(mr.rating IS NULL, 'pending', mr.rating) AS rating 
FROM member_points AS m
JOIN members AS c1 ON m.id_from = c1.id
JOIN members AS c2 ON m.id_to = c2.id
    and m.id_to='.$_SESSION["userid"].'
LEFT JOIN member_ratings AS mr ON mr.id_from = c1.id
    AND mr.id_to = c2.id
GROUP BY name_from';

如果这不是您需要的，请告诉我，我会尽力帮助您。