1

背景资料

系统

操作系统：

bash$ lsb_release -a
No LSB modules are available.
Distributor ID: Debian
Description:    Debian GNU/Linux 6.0.5 (squeeze)
Release:        6.0.5
Codename:       squeeze

MySQL：

bash$ mysql --version
mysql  Ver 14.14 Distrib 5.1.63, for debian-linux-gnu (x86_64) using readline 6.1

数据库

引擎：MyISAM

表：post_votes

mysql> describe post_votes;
+-----------+---------------------+------+-----+---------+----------------+
| Field     | Type                | Null | Key | Default | Extra          |
+-----------+---------------------+------+-----+---------+----------------+
| id        | bigint(20) unsigned | NO   | PRI | NULL    | auto_increment |
| post_uuid | binary(16)          | YES  |     | NULL    |                |
| user_uuid | binary(16)          | YES  |     | NULL    |                |
| vote      | tinyint(4)          | YES  |     | 0       |                |
+-----------+---------------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)

表：帖子

mysql> describe posts;
+-------------+---------------------+------+-----+----------------------+----------------+
| Field       | Type                | Null | Key | Default              | Extra          |
+-------------+---------------------+------+-----+----------------------+----------------+
| id          | bigint(20) unsigned | NO   | PRI | NULL                 | auto_increment |
| post_uuid   | binary(16)          | YES  | UNI | NULL                 |                |
| owner_uuid  | binary(16)          | YES  |     | NULL                 |                |
| created     | int(10) unsigned    | YES  |     | NULL                 |                |
| edited      | int(10) unsigned    | YES  |     | NULL                 |                |
| title       | varchar(50)         | YES  |     | NULL                 |                |
| description | varchar(150)        | YES  |     | NULL                 |                |
| link        | varchar(100)        | YES  |     | NULL                 |                |
| properties  | varchar(20)         | YES  |     | 00000000000000000000 |                |
+-------------+---------------------+------+-----+----------------------+----------------+
9 rows in set (0.00 sec)

问题

我想根据他们的赞成/反对比率来选择帖子。我想出了一个计算赞成票和反对票但似乎无法将其加入实际帖子的查询。

工作（不完整）查询

(
    SELECT COUNT(*), post_uuid, 'upvotes' AS 'mode'
    FROM post_votes a
    WHERE a.vote = 1
    GROUP BY post_uuid
)
UNION
(
    SELECT COUNT(*), post_uuid, 'downvotes' AS 'mode'
    FROM post_votes a
    WHERE a.vote = -1
    GROUP BY post_uuid
)

损坏的查询

(
    (
        SELECT COUNT(*), post_uuid, 'upvotes' AS 'mode'
        FROM post_votes a
        WHERE a.vote = 1
        GROUP BY post_uuid
    )
    UNION
    (
        SELECT COUNT(*), post_uuid, 'downvotes' AS 'mode'
        FROM post_votes a
        WHERE a.vote = -1
        GROUP BY post_uuid
    )
)
a
LEFT JOIN posts b ON a.post_uuid = b.post_uuid

我究竟做错了什么？我得到的错误信息是：

ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that
corresponds to your MySQL server version for the right syntax to use near 'UNION
(
SELECT COUNT(*), post_uuid, 'downvotes' AS 'mode'
4

1 回答 1

2

该UNION对实际上是一个派生表，并且必须出现在FROM子句中才能参与JOIN. 由于它现在在FROM子句中，因此您必须使用SELECT它的列，否则您的查询根本没有SELECT子句。MySQL 的语法是宽松的，但不会宽松到允许缺少SELECT子句。

/* SELECT the cols produced by the UNION */
SELECT 
   a.`num`, 
   a.`post_uuid`,
   a.`mode`,
  /* And other columns from `posts` if needed */
   b.`something`,
   b.`something_else`,
   b.`some_other_thing`
/* UNIONs are a derived table, belonging in FROM */
FROM (
    (
        SELECT COUNT(*) as num, post_uuid, 'upvotes' AS 'mode'
        FROM post_votes a
        WHERE a.vote = 1
        GROUP BY post_uuid
    )
    UNION
    (
        SELECT COUNT(*) num, post_uuid, 'downvotes' AS 'mode'
        FROM post_votes a
        WHERE a.vote = 1
        GROUP BY post_uuid
    )
) a
LEFT JOIN posts b ON a.post_uuid = b.post_uuid

回顾一下，我相信您可以UNION通过使用SUM(CASE)聚合模式来简化：

SELECT 
  b.post_uuid,
  SUM(CASE WHEN a.vote = 1 THEN 1 ELSE 0 END) AS upvotes,
  /* Assume -1 for downvotes? */
  SUM(CASE WHEN a.vote = -1 THEN 1 ELSE 0 END) AS downvotes
FROM
  posts b
  LEFT JOIN post_votes a ON a.post_uuid = b.post_uuid
GROUP BY b.post_uuid
于 2012-11-26T00:03:56.833 回答