SQL select - 包括不存在的行？

Question

有人可以帮忙解决这个问题吗？

SELECT A.post_id, B.post_id, A.ul_value as "likes", B.ul_value as "dislikes" 
FROM wp_like_dislike_counters as A, 
wp_like_dislike_counters as B 
where 
A.post_id = B.post_id 
and A.ul_key = 'u_like' 
and B.ul_key = 'u_dislike' 

我得到的结果：

3, 3, 1, 2 
4, 4, 3, 2 

我想要的结果：

3, 3, 1, 2 
4, 4, 3, 2 
1, 1, 0, 1 

下面是表格中的数据。请注意，id为32的行的post_id为1，而l_value为1（表示1不喜欢）。由于这个post_id没有＆＃34; like＆＃34;的行，我想在上面想要的结果中显示为0。这种表结构可以实现吗？

id | post_id | l_key     | l_value 
---+---------+-----------+---------
35 |       3 | u_dislike | 2 
34 |       4 | u_dislike | 2 
31 |       4 | u_like    | 3 
32 |       1 | u_dislike | 1 
36 |       3 | u_like    | 1

Answer 1

您希望使用条件聚合而不是连接来执行此操作：

select post_id,
       sum(case when ul_key = 'u_like' then ul_value else 0 end) as likes,
       sum(case when ul_key = 'u_dislike' then ul_value else 0 end) as dislikes
from wp_like_dislike_counters
group by post_id;