MySQL：返回每行几个项目的计数

Question

我有一张列出一些项目的表格。
然后我有 3 个不同的表，每个表都记录了针对某个项目执行的各种操作，以及日期和执行者：

Table "items":
item_id, title, description

Table "item_views":
item_id, date, user_id

Table "item_follows":
item_id, date, user_id

Table "item_likes":
item_id, date, user_id

我正在尝试提出一个查询，该查询将列出每个项目，以及在其他 3 个表中的每个表中引用它们的次数。

它只需要计算两个给定日期之间的操作，而且我还需要能够按任何计数器排序。

所以我会给它两个日期并说“按喜欢排序”，它会返回如下内容：

item id     item title     views_counter    follows_counter      likes_counter
item1       Some title     0                5                    6
item2       Some title     4                0                    4
item3       Some title     3                2                    0

任何建议将不胜感激，在此先感谢！

Answer 1

您可以使用union all和聚合：

select i.*,
       sum(which = 'view') as views,
       sum(which = 'follow') as follows,
       sum(which = 'like') as likes
from items i left join
     ((select item_id, 'view' as which
       from item_views
      ) union all
      (select item_id, 'follow' as which
       from item_follows
      ) union all
      (select item_id, 'like' as which
       from item_likes
      )
     ) vfl
     on i.item_id = vfl.item_id
group by i.item_id;

Answer 2

对查找计数的子查询使用左连接：

SELECT
    i.item_id,
    i.title,
    COALESCE(v.view_cnt, 0)   AS views_counter,
    COALESCE(f.follow_cnt, 0) AS follows_counter,
    COALESCE(l.like_cnt, 0)   AS likes_counter
FROM items i
LEFT JOIN
(
    SELECT item_id, COUNT(*) AS view_cnt
    FROM item_views
    GROUP BY item_id
) v
    ON v.item_id = i.item_id
LEFT JOIN
(
    SELECT item_id, COUNT(*) AS follow_cnt
    FROM item_follows
    GROUP BY item_id
) f
    ON f.item_id = i.item_id
LEFT JOIN
(
    SELECT item_id, COUNT(*) AS like_cnt
    FROM item_likes
    GROUP BY item_id
) l
    ON l.item_id = i.item_id