[英]Mysql Best way to query notifications like on stackoverflow
我有4张桌子:
comments
+----+-----------+--------------+-------+
| id | content | user_id | article_id |
+----+-----------+--------------+-------+
| 1 | Comment 1 | 2 | 5 |
| 2 | Comment 2 | 5 | 3 |
| 3 | Comment 3 | 1 | 6 |
| 4 | Comment 4 | 6 | 8 |
| 5 | Comment 5 | 1 | 6 |
| ...| ... | ... | ... |
+----------------+---------+------------+
votes
+----+----------+--------------+---+
| id | type | user_id | article_id |
+----+----------+--------------+---+
| 1 | 1 | 2 | 5 |
| 2 | 1 | 3 | 3 |
| 3 | 0 | 1 | 6 |
| 4 | 1 | 7 | 4 |
| 5 | 0 | 9 | 4 |
| 6 | 0 | 1 | 6 |
| ...| ... | ... | ... |
+------------+----------+----------+
notifications (object_id is the id of the vote|comment)
+----+----------+--------------+-------------+-------------+--------------+
| id | object_url| object_id |activitytype_id| sender_id | recipient_id |
+----+----------+------------+---------------+-------------+--------------+
| 1 | /../../.. | 1 | 2 | 2 | 6 |
| 2 | /../../.. | 2 | 2 | 3 | 2 |
| 3 | /../../.. | 1 | 1 | 2 | 7 |
| 3 | /../../.. | 2 | 1 | 5 | 2 |
| 3 | /../../.. | 3 | 1 | 1 | 3 |
| 3 | /../../.. | 3 | 3 | 1 | 2 |
| 3 | /../../.. | 4 | 2 | 7 | 8 |
| 3 | /../../.. | 5 | 3 | 9 | 1 |
| 3 | /../../.. | 6 | 3 | 1 | 5 |
| ...| ... | ... | ... | | |
+----+-----------+-----------+---------------+-------------+--------------+
activitytypes
+----+------------+
| id | label |
+----+------------+
| 1 | comment |
| 2 | vote up |
| 3 | vote down |
| ...| ... |
+-----------------+
我想收到类似stackoverflow的通知。
我想查询特定用户的每种活动类型和object_url组合的最后通知(如果活动类型为注释,则带有注释内容,否则为null)。
例如,我有3个飞行器A,B和C,都具有3条评论,4个投票和2个投票。 如何获得每篇文章的最后评论,投票和投票?
我已经尝试过以下查询:
SELECT n.id, n.object_url , n.object_id, n.activitytype_id, IF(n.activitytypeId = 1,
(SELECT content FROM comments WHERE id=n.object_id), null) AS activitycontent
FROM notifications n WHERE n.recipient_id =1
GROUP BY n.activitytype_id,n.object_url
ORDER BY n.id DESC
但这是行不通的。 有人可以帮忙吗?
编辑:在farhadamjady的答案中以下查询给了我第一条评论:
SELECT
n.id,
n.object_url,
n.object_id,
n.activitytype_id,
cm.content AS activitycontent
FROM
notifications n
LEFT OUTER JOIN `COMMENT` AS cm ON cm.id = n.object_id and n.activitytypeId = 1
WHERE
n.recipient_id = 1
GROUP BY
n.activitytype_id,
n.object_url
HAVING MAX(cm.id)
ORDER BY
n.id DESC
如何更改它以获取最后一个?
您应该像这样使用左外部连接:
SELECT
n.id,
n.object_url,
n.object_id,
n.activitytype_id,
cm.content AS activitycontent
FROM
notifications n
LEFT OUTER JOIN `COMMENT` AS cm ON cm.id = n.object_id and n.activitytypeId = 1
WHERE
n.recipient_id = 1
GROUP BY
n.activitytype_id,
n.object_url
HAVING MAX(cm.id)
ORDER BY
n.id DESC
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