如何加入多重表? [英] How do I join multible tables?

问题描述

我有下表(和示例值):

I have the following tables (and example values):

**user:**
user_id (1, 2, 3)
username (john33, reddiamond...)
password (pass1, pass2...)

**session:**
session_id (4,5, 6)
user_id (1, 2, 3)

**activity**
activity_id (1, 2)
name (running, walking...)

**user_activity**
user_activity_id (1, 2, 3, 4, 5)
session_id (4, 5)
activity_id (1, 2)

具有相同名称的所有列都相关.在表user_activity中，有几行描述了会话的活动，而该活动指的是用户.

All columns with the same name are related. In the table user_activity there are rows which describe what is the session's activity and the activity refers to users.

但是我想获得一张表，该表描述了用户当前正在做的事情:

However I would like to get the table which describes what the user is currently doing:

**result**
username(john33)
activity.name(walking)

获取结果表的SQL语句是什么?

What is the SQL statement that gets the result table?

(我正在使用MSSQL).

(I'm using MSSQL).

推荐答案

SELECT u.username, a.name
FROM user_activity ua
INNER JOIN session s
ON ua.session_id = s.session_id
INNER JOIN user u
ON s.user_id = u.user_id
INNER JOIN activity a
ON ua.activity_id = a.activity_id

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