Question

如何将此查询实施到更复杂的查询中？

SELECT count(`p`.`id`) as count 
FROM `wallPosts` `p` LEFT JOIN `wallPosts` `c` on `c`.`parentID` = `p`.`id` 
where `p`.`parentID` is not null group by `p`.`parentID`

返回：

+-------+
| count |
+-------+
|     3 |
|     2 |
+-------+

我已将“复杂”查询简化为：

SELECT `p`.`id`, 
       `p`.`parentid`, 
       `p`.`commenterid`, 
       `p`.`userid`, 
       `p`.`post`, 
       `p`.`date`, 
       `p`.`tags`, 
       CASE 
         WHEN ( `p`.`userid` = `p`.`commenterid` 
                AND `p`.`parentid` IS NULL ) THEN 'true' 
         ELSE 'false' 
       end AS isMain 
FROM   `wallposts` `p` 
       LEFT JOIN `wallposts` `c` 
              ON `c`.`parentid` = `p`.`id` 
       LEFT JOIN `users` 
              ON `users`.`id` = `p`.`commenterid` 
GROUP  BY `p`.`id` 
HAVING `ismain` = 'true' 
ORDER  BY `p`.`date` DESC 
LIMIT  20

返回：

+----+----------+-------------+--------+--------+---------------------+------+--------+
| id | parentID | commenterID | userID |  post  |        date         | tags | isMain |
+----+----------+-------------+--------+--------+---------------------+------+--------+
|  5 | NULL     |           1 |      1 | post#1 | 2013-07-26 13:29:02 | NULL | true   |
|  1 | NULL     |           1 |      1 | post#2 | 2013-07-26 13:28:23 | NULL | true   |
+----+----------+-------------+--------+--------+---------------------+------+--------+

这就是我想要的：

+----+----------+-------------+--------+--------+---------------------+------+--------+-------+
| id | parentID | commenterID | userID |  post  |        date         | tags | isMain | count |
+----+----------+-------------+--------+--------+---------------------+------+--------+-------+
|  5 | NULL     |           1 |      1 | post#1 | 2013-07-26 13:29:02 | NULL | true   |     3 |
|  1 | NULL     |           1 |      1 | post#2 | 2013-07-26 13:28:23 | NULL | true   |     2 |
+----+----------+-------------+--------+--------+---------------------+------+--------+-------+

这是我的全桌：

+----+----------+-------------+--------+---------+------+---------------------+
| id | parentID | commenterID | userID |  post   | tags |        date         |
+----+----------+-------------+--------+---------+------+---------------------+
|  1 | NULL     |           1 |      1 | post#1  | NULL | 2013-07-26 13:28:23 |
|  2 | 1        |           1 |      1 | reply#1 | NULL | 2013-07-26 13:28:28 |
|  3 | 1        |           1 |      1 | reply#2 | NULL | 2013-07-26 13:28:38 |
|  4 | 1        |           1 |      1 | reply#3 | NULL | 2013-07-26 13:28:54 |
|  5 | NULL     |           1 |      1 | post#2  | NULL | 2013-07-26 13:29:02 |
|  6 | 5        |           1 |      1 | reply#1 | NULL | 2013-07-26 13:29:05 |
|  7 | 5        |           1 |      1 | reply#2 | NULL | 2013-07-26 13:29:06 |
+----+----------+-------------+--------+---------+------+---------------------+

正如您所看到的，我正在尝试计算每个帖子的回复数量。

感谢您的帮助！

Answer 1

我已经就类似的问题给你答案了。请参考http://sqlfiddle.com/#!2/9a6cc/9。可能对你有用

SELECT `p`.`id`, 
       `p`.`parentid`, 
       `p`.`commenterid`, 
       `p`.`userid`, 
       `p`.`post`, 
       `p`.`date`, 
       `p`.`tags`, 
       CASE 
         WHEN ( `p`.`userid` = `p`.`commenterid` 
                AND `p`.`parentid` IS NULL ) THEN 'true' 
         ELSE 'false' 
       end AS isMain ,
        cnt.count1
FROM   `wallposts` `p` 
       LEFT JOIN `wallposts` `c` 
              ON `c`.`parentid` = `p`.`id` 
       LEFT JOIN `users` 
              ON `users`.`id` = `p`.`commenterid` 
       LEFT JOIN (SELECT count(`p`.`id`) as count1,parentid FROM `wallPosts` `p` where parentid is not null group by parentid) cnt ON 
                  cnt.parentid = p.id
GROUP  BY `p`.`id` 
HAVING `ismain` = 'true' 
ORDER  BY `p`.`date` DESC 
LIMIT  20 ;

Answer 2

搞定了：

SELECT

`p`.`id`,
`p`.`parentID`,
`p`.`commenterID`,
`p`.`userID`,
`p`.`post`,
`p`.`date`,
`p`.`tags`,
`commentCount`.`count`,

case
when (`p`.`userID` = `p`.`commenterID` and `p`.`parentID` IS NULL)
then 'true'
else 'false'
end as isMain

FROM `wallPosts` `p`

LEFT JOIN `wallPosts` `c` ON `c`.`parentID` = `p`.`id`
LEFT JOIN `users` ON `users`.`id` = `p`.`commenterID`

LEFT JOIN (
  SELECT `parentID`, COUNT(*) AS count
  FROM `wallPosts` where `parentID` is not null
  GROUP BY `parentID`
) AS `commentCount`

ON `p`.`id` = `commentCount`.`parentID`


GROUP BY `p`.`id`

having `isMain` = 'true'

ORDER BY `p`.`date` DESC

limit 20

子查询中的select或count内的mysql计数

2 个答案: