计算每个人分配给的活动

Question

我有一个问题，我需要能够计算孩子已注册要做的活动数量。目前，我有这个：

SELECT child_fname, activity_id
FROM Child
LEFT JOIN Activity
ON Activity.ActivityID=activity.activity_id
ORDER BY Child.child_fname;

我的桌子是这样的：

子表

• child_firstname
• child_secondname
• child_age

活动

• activity_id
• activity_name
• activity_price

ChildActivity

• child_id
• activity_id

---

我正在寻找类似的东西

• [姓名] [活动数量]
• Sam Smith 5
• 瑞秋·史密斯4

我尝试使用JOIN，但我的查询没有更新新数据

Answer 1

使用INNER JOIN

SELECT Child.child_firstname, COUNT(Activity.activity_id)
FROM Child
INNER JOIN Activity ON Activity.activity_id=ChildActivity.activity_id
INNER JOIN ChildActivity ON ChildActivity.child_id=Child.child_id
ORDER BY Child.child_firstname;

Ps：请检查列名和表格。

示例 SQL Inner-join with 3 tables?

Answer 2

这就是我想要的

SELECT child_fname as Name, count(activity_id) as `Number of Activities`
FROM Child
LEFT JOIN ChildActivity
ON ChildActivity.child_id=Child.child_id
GROUP BY Child.child_id
ORDER BY COUNT(activity_id) DESC;

重点是您想将Child与ChildActivity一起加入并按Child分组，这样，Child中的每条记录都会得到一个活动计数。