PostgreSQL GROUP BY与MySQL不同？

ERROR: column “XYZ” must appear in the GROUP BY clause or be used in
an aggregate function

有人能告诉我我做错了什么吗？

MySQL 100％工作：

SELECT `availables`.*
FROM `availables`
INNER JOIN `rooms` ON `rooms`.id = `availables`.room_id
WHERE (rooms.hotel_id = 5056 AND availables.bookdate BETWEEN '2009-11-22' AND '2009-11-24')
GROUP BY availables.bookdate
ORDER BY availables.updated_at

PostgreSQL错误：

ActiveRecord::StatementInvalid: PGError: ERROR: column
“availables.id” must appear in the GROUP BY clause or be used in an
aggregate function:
SELECT “availables”.* FROM “availables” INNER
JOIN “rooms” ON “rooms”.id = “availables”.room_id WHERE
(rooms.hotel_id = 5056 AND availables.bookdate BETWEEN E’2009-10-21′
AND E’2009-10-23′) GROUP BY availables.bookdate ORDER BY
availables.updated_at

生成SQL的Ruby代码：

expiration = Available.find(:all,
    :joins => [ :room ],
    :conditions => [ "rooms.hotel_id = ? AND availables.bookdate BETWEEN ? AND ?", hostel_id, date.to_s, (date+days-1).to_s ],
    :group => 'availables.bookdate',
    :order => 'availables.updated_at')  

预期输出(来自MySQL查询)：

+-----+-------+-------+------------+---------+---------------+---------------+
| id  | price | spots | bookdate   | room_id | created_at    | updated_at    |
+-----+-------+-------+------------+---------+---------------+---------------+
| 414 | 38.0  | 1     | 2009-11-22 | 1762    | 2009-11-20... | 2009-11-20... |
| 415 | 38.0  | 1     | 2009-11-23 | 1762    | 2009-11-20... | 2009-11-20... |
| 416 | 38.0  | 2     | 2009-11-24 | 1762    | 2009-11-20... | 2009-11-20... |
+-----+-------+-------+------------+---------+---------------+---------------+
3 rows in set

MySQL的：

SELECT a,b,c,d,e FROM table GROUP BY a

这为每个a值提供了1行(哪一个,你真的不知道).实际上你可以猜到,因为MySQL不知道哈希聚合,所以它可能会使用排序…但它只会对a进行排序,因此行的顺序可能是随机的.除非它使用多列索引而不是排序.好吧,无论如何,它没有被查询指定.

Postgres的：

SELECT DISTINCT ON (a) a,b,c,d,e FROM table ORDER BY a,b,c

这为a的每个值提供了1行,该行将是根据查询指定的ORDER BY排序中的第一行.简单.

请注意,这里,它不是我正在计算的聚合.因此GROUP BY实际上毫无意义. DISTINCT ON更有意义.

Rails与MySQL结合,所以我并不惊讶它生成的SQL在Postgres中不起作用.