这是我的表结构：
-- reputations
+----+-------------+---------+-------+------------+------------+
| id | post_id | user_id | score | reputation | date_time |
+----+-------------+---------+-------+------------+------------+ -- Suppose:
| 1 | 1 | 1 | 1 | 5 | 1500489844 | -- out of last week
| 2 | 4 | 3 | -1 | -2 | 1500499815 | -- out of last week
| 3 | 2 | 3 | 1 | 5 | 1500584821 |
| 4 | 3 | 1 | 1 | 5 | 1501389166 |
| 5 | 2 | 4 | 1 | 5 | 1501399142 |
| 6 | 2 | 1 | -1 | -2 | 1501399142 |
| 7 | 4 | 1 | 0 | 15 | 1501481186 |
| 8 | 5 | 1 | 1 | 5 | 1501481297 |
+----+-------------+---------+-------+------------+------------+
-- Note: the last row came from an accepted-answer, that's why its score is 0
-- post_tag
+---------+--------+
| post_id | tag_id |
+---------+--------+
| 1 | 2 |
| 1 | 4 |
| 2 | 2 |
| 3 | 1 |
| 3 | 4 |
| 4 | 3 |
| 5 | 1 |
+---------+--------+
-- tags
+----+--------+
| id | name |
+----+--------+
| 1 | php |
| 2 | html |
| 3 | css |
| 4 | mysql |
+----+--------+
这是我的疑问：
SELECT
t.tag, sum(r.reputation) AS tag_reputation, sum(r.score) AS tag_score
FROM
users u
LEFT JOIN reputations r
ON r.user_id = u.id
AND r.date_time > 1500584821
JOIN post_tag pt ON pt.post_id = r.post_id
JOIN tags t ON t.id = pt.tag_id
WHERE u.id = 1 -- Specific user: Jack
GROUP BY
u.id, u.user_name, t.tag
ORDER BY
u.id, tag_reputation DESC;
以下是目前的结果：
tag | tag_reputation | tag_score
----: | :------------- | :---------
css | 15 | 0
php | 10 | 2
mysql | 5 | 1
html | -2 | -1
如您所见，结果是一个标签列表，其中包含按tag_reputation排序的特定用户的信誉和分数。的 Fiddle
现在我还想计算每个标签的帖子数量。所以这是预期结果：
tag | tag_reputation | tag_score | post_num
----: | :------------- | :--------- | :-------
css | 15 | 0 | 1
php | 10 | 2 | 2
mysql | 5 | 1 | 1
html | -2 | -1 | 1
我该怎么做？我想我必须处理post_id列和GROUP BY子句。但我不知道究竟是怎么回事。