我有一个存储用户投票的表,如下所示:
+----+---------+-------+---------+
| id | post_id | value | user_id |
+----+---------+-------+---------+
| 1 | 103 | 1 | 1 |
| 2 | 105 | 1 | 3 |
| 3 | 106 | 1 | 1 |
| 4 | 108 | 0 | 1 |
| 5 | 108 | 0 | 2 |
| 6 | 105 | 0 | 2 |
| 7 | 105 | 1 | 1 |
+----+---------+-------+---------+
id
是投票ID,post_id
是帖子ID,value
是类似/不同的布尔值(1 =赞,0 =不同),user_id
是选民ID。
我想得到一个列表:
所以我写了这个查询:
SELECT
post_id,
COUNT(
CASE
WHEN value = 1
THEN 1
END
) AS likes,
COUNT(
CASE
WHEN value = 0
THEN 1
END
) AS unlikes,
(CASE
WHEN user_id = 1 -- I choose user_id manually
THEN id
END) AS vote_id,
(CASE
WHEN user_id = 1
THEN value
END) AS user_vote
FROM votes
GROUP BY post_id;
我的期望是这样的:
+---------+-------+---------+---------+-----------+
| post_id | likes | unlikes | vote_id | user_vote |
+---------+-------+---------+---------+-----------+
| 103 | 1 | 0 | 1 | 1 |
| 105 | 2 | 1 | 7 | 1 |
| 106 | 1 | 0 | 3 | 1 |
| 108 | 0 | 2 | 4 | 0 |
+---------+-------+---------+---------+-----------+
但这是我的结果:
+---------+-------+---------+---------+-----------+
| post_id | likes | unlikes | vote_id | user_vote |
+---------+-------+---------+---------+-----------+
| 103 | 1 | 0 | 1 | 1 |
| 105 | 2 | 1 | NULL | NULL |
| 106 | 1 | 0 | 3 | 1 |
| 108 | 0 | 2 | 4 | 0 |
+---------+-------+---------+---------+-----------+
由于我的查询的最后一行(NULL
),会发生GROUP BY post_id
个值。如果表格中存在重复的post_id
(例如,如果某个其他用户已经为该帖子投票),则CASE
会忽略它并返回NULL
。
我该怎么办?
答案 0 :(得分:3)
您的查询存在的问题是最后两列缺少聚合。假设用户只能为帖子投票一次,您可以使用MAX()
(或MIN()
)安全地组合这些值,因为最多只有一个匹配的行:
SELECT v.post_id, SUM(value = 1) AS likes, SUM(value = 0) AS unlikes,
MAX(CASE WHEN user_id = 1 THEN vote_id END) AS user1_voteid,
MAX(CASE WHEN user_id = 1 THEN value END) AS user1_vote
FROM votes v
GROUP BY v.post_id;