需要查询帮助——聚合和多重连接

Question

我的应用程序中使用了几个表。一个维护产品列表，另一个维护对这些项目的评论，另一个包含这些项目的星级评级，最后一个包含这些项目的购买情况。我的表看起来像这样：

tbl_item:
---------
id      INT (primary key)
name    VARCHAR (product name)

tbl_comment:
------------
id          INT (primary key)
item_id     INT (foregin key -> tbl_item.id)
commenttext VARCHAR

tbl_rating:
-----------
id          INT (primary key)
item_id     INT (foreign key -> tbl_item.id)
rating      DOUBLE

tbl_purchases:
--------------
id          INT (primary key)
item_id     INT (foreign key -> tbl_item.id)

我想执行一个返回以下内容的查询：

* The design ID
* The average rating
* The number of comments
* The number of purchases

我有与此类似的内容，但它返回不正确的数据：

SELECT  d.id ,
        COUNT(tbl_purchases.id) AS purchase_count, 
        COUNT(tbl_comment.id) AS comment_count,
        AVG(tbl_rating.rating) AS item_rating,
    FROM tbl_item d
    LEFT JOIN tbl_purchases ON tbl_purchases.item_id = d.id
    LEFT JOIN tbl_comment ON tbl_comment.item_id = d.id
    LEFT JOIN tbl_rating ON tbl_rating.id = d.id
    GROUP BY d.id;

我发现我的 COUNT() 列返回相同的值两列，这绝对是不正确的。显然我在连接或 GROUP BY 中做错了什么，但我不完全确定是什么。我是一个 Java 人员，而不是 SQL 人员，所以我不确定这个 SELECT 语句出了什么问题。

谁能帮助我构建这个查询？有没有办法以这种方式跨多个不同的表执行聚合查询？谢谢！！

Answer 1

试试这个：

SELECT  d.id ,
        COALESCE(t.purchase_count,0) as purchase_count, 
        COALESCE(c.comment_count,0) as comment_count,
        r.item_rating,
    FROM tbl_item d
    LEFT JOIN (SELECT item_id, COUNT(1) as purchase_count from tbl_purchases group by item_id) as t on t.item_id = d.id
    LEFT JOIN (SELECT item_id, COUNT(1) as comment_count from tbl_comment group by item_id) as c ON c.item_id = d.id
    LEFT JOIN (SELECT item_id, AVG(rating) as item_rating from tbl_rating group by item_id) as r ON r.item_id = d.id;

Answer 2

使用 count(distinct(tbl_purchases.id)) 应该可以解决您的问题，而无需其他人提供的更复杂的查询（但也是正确的）查询。

Answer 3

这在一定程度上取决于你使用的数据库，但这在 PostgreSQL 中不起作用：

    SELECT d.id , p.count, c.count, AVG(I.rating)
      FROM tbl_item d
      JOIN ( SELECT count(id), item_id as id from tbl_purchases ) as P
     USING (id)
      JOIN ( SELECT count(id), item_id as id from tbl_comment ) as C
     USING (id)
 LEFT JOIN tbl_rating as I
        ON tbl_rating.id = d.id
  GROUP BY d.id
;