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。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

我有类似的东西，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（）列为两列返回相同的值，这绝对不正确。很明显，我在我的连接或GROUP BY中做错了，但我不完全确定是什么。我是一个Java人，而不是SQL人，所以我不确定这个SELECT语句中出了什么问题。

任何人都可以帮我构建这个查询吗？有没有办法以这种方式跨多个不同的表执行此聚合查询？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))可以解决您的问题，而无需其他人提供的更复杂的查询（但也更正确）。

Answer 3

它在某种程度上取决于你正在使用什么数据库，但这在PostgreSQL中有用：

    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
;

需要查询帮助 - 聚合和多个联接

3 个答案: