SQL - 在SUM＆amp;中遇到麻烦带连接表的AVG查询

Question

我有两张桌子，一张是评级＆amp;第二个是property_listing，我正在考虑SUM＆amp; AVG基于这个字段safety, entertainment, parking, public_transportation, community_centers，我创建了这个SQL查询：

SELECT property_listing.*, coalesce( avg( rating.safety + rating.entertainment + rating.parking + rating.public_transportation + rating.community_centers )/5 , 0 ) AS rate
FROM property_listing
JOIN rating ON property_listing.property_id = rating.property_id
ORDER BY property_listing.property_id DESC

但是在运行这个SQL之后我只得到一行。为什么我没有用AVG获取其他记录？

我的SQL小提琴：http://sqlfiddle.com/#!2/b2ab0/2

[Table 1: rating]

review_id property_id user_id safety entertainment parking public_transportation community_centers
--------- ----------- ------- ------ ------------- ------- --------------------- -----------------
8         553         24      2      4             5       5                     5
9         552         24      3      4             5       5                     5
10        550         24      3      4             5       5                     5
11        523         24      2      4             5       5                     5


[Table 2: property_listing]

property_id title                           user_id property_type_id
----------- ------------------------------- ------- ----------------
523         Bograshov St Tel Aviv Israel IL 48      16
550         Bograshov St Tel Aviv Israel IL 54      13
552         Bograshov St Tel Aviv Israel IL 54      16
553         Bograshov St Tel Aviv Israel IL 48      16

[Wanting this result with AVG]

property_id title                           user_id property_type_id rating
----------- ------------------------------- ------- ---------------- ------
523         Bograshov St Tel Aviv Israel IL 48      16               4.2
550         Bograshov St Tel Aviv Israel IL 54      13               4.4
552         Bograshov St Tel Aviv Israel IL 54      16               4.4
553         Bograshov St Tel Aviv Israel IL 48      16               4.2

Answer 1

尝试使用group by子句，因为avg()是一个聚合函数，您需要按列对结果进行分组：

SELECT property_listing.*,
       COALESCE(Avg(rating.safety + rating.entertainment
                    + rating.parking
                    + rating.public_transportation
                    + rating.community_centers) / 5, 0) AS rate
FROM   property_listing
       JOIN rating
         ON property_listing.property_id = rating.property_id
GROUP  BY property_listing.property_id
ORDER  BY property_listing.property_id DESC 

Answer 2

以下内容应该有效：

SELECT property_listing.property_ID
,      property_listing.title
,      property_listing.user_ID
,      property_listing.Property_Type_ID
,      avg( rating.safety + rating.entertainment + rating.parking + rating.public_transportation + rating.community_centers )/5 AS rate
FROM property_listing
JOIN rating ON property_listing.property_id = rating.property_id
group by property_listing.property_ID
,      property_listing.title
,      property_listing.user_ID
,      property_listing.Property_Type_ID
ORDER BY property_listing.property_id DESC

我不确定为什么你的查询确实没有用，但是在mysql中你不必指定组，我不能说它如何评估这样的查询。指定您的小组总是更好，这样您才能知道实际发生了什么。

我删除了合并，但如果您愿意，可以重新添加。