mySQL 的完全新手。所以任何帮助将不胜感激。
我有 3 个表 -- carts
、users
、actions
。
carts:
+------------+-------------+-------+
| cartId | session_id | userId|
+------------+-------------+-------+
users:
+----------+-------------+
| usedId | email |
+----------+-------------+
actions:
+-------------+------------------+---- ---------+
| session_id | impressionAction | impressionId |
+-------------+------------------+-----+--------+
在carts
中,每一行有一个session_id
。
在users
中,每一行有一个userId
。
在 actions
中,每个 session_id
有多行,用于计算该 session 的所有操作。
我想JOIN
三个表得到类似的输出
+------+-------------+--------+------------------+--------------+-------+
userId | session_id | cartId | impressionAction | impressionId | email |
+------+-------------+--------+------------------+--------------+-------+
每个 userId
和 session_id
有多行;本质上是一个扁平化的文件。我认为如果我们在 userId
上 JOIN
carts
和 users
导致说 A
和然后 JOIN
A
和 actions' on
session_id`,我们到家了。
样本预期输出是:
+------------+-------------+--------+------------------+--------------+---------+
userId | session_id | cartId | impressionAction | impressionId | email |
+------------+-------------+--------+------------------+--------------+---------+
| 1234 | abc3f45 | 0001 | LOGIN | 2032 |ab@yc.com|
| 1234 | abc3f45 | 0001 | ADD | 4372 |ab@yc.com|
| 1234 | abc3f45 | 0001 | ADD | 4372 |ab@yc.com|
| 1234 | abc3f45 | 0001 | SENDMAIL | ab@yc.com |ab@yw.com|
| 4567 | def4rg4 | 0002 | LOGIN | 2032 |db@yw.com|
| 4567 | def4rg4 | 0002 | ADD | 4372 |db@yw.com|
| 4567 | def4rg4 | 0002 | REMOVE | 3210 |db@yw.com|
+------------+-------------+--------+------------------+--------------+---------+**
我不知道如何在没有一个公共(public)键的情况下连接 3 个表。我什至不知道它叫什么类型的连接。
本质上,我们试图连接 3 个具有非重叠键的表,通过第一个 JOIN
收集一个公共(public)键,然后将中间键与第三个连接。这称为 CROSS JOIN
吗?如果没有,有名字吗?
最佳答案
摘自你上面的评论
A USER may select many products, add them to their CART; a single USER may have multiple CARTS and at the end of the event, they can EMAIL the cart to themselves; the ACTIONS of the user are stored in the actions table
这就是我对结构的看法(考虑到您的数据)
+---------------------+ +---------------------+ +---------------------+
| users | | carts | | actions |
+---------------------+ +---------------------+ +---------------------+
| user_id [PK] |--| | cart_id [PK] | | impression_id [PK] |
| email | |--| user_id [FK] | | action_name |
| | | product_id [FK] | |--| session_id [FK]* |
+---------------------+ | session_id [FK]* |--| | |
| | +---------------------+
+---------------------+
正如您在上面看到的,我首先加入购物车,然后加入操作,因为只有桌面购物车同时具有用户和 session 数据。
购物车和操作上 session_id 旁边的 [FK]*
可能看起来是外键,但在这种情况下它不是 - 因为没有单独的 session 表将其放置为PK(主键)
您询问了join
- 它与inner join
相同。 INNER JOIN 通过基于连接谓词组合两个表(A 和 B)的列值来创建一个新的结果表。该查询将 A 的每一行与 B 的每一行进行比较,以找到满足连接谓词的所有行对。
这是一个可能的表格内容
+------------------------+
| users |
+------------------------+
| id | email |
+------+-----------------+
| 1 | first@mail.org |
| 2 | second@mail.org |
| 3 | third@mail.org |
+------+-----------------+
+------------------------------------------+
| carts |
+------------------------------------------+
| id | user_id | product_id | session_id |
+------+---------+------------+------------+
| 1 | 1 | 5 | 1aaaa |
| 2 | 2 | 5 | 2ffff |
| 3 | 3 | 8 | 3ddddd |
| 4 | 1 | 5 | 1aaaaa |
| 5 | 3 | 9 | 3bbbbb |
| 6 | 1 | 6 | 1ccccc |
+------+---------+------------+------------+
+-------------------------------+
| actions |
+-------------------------------+
| id | name | session_id |
+------+-----------+------------+
| 1 | ADD | 1aaaa |
| 2 | ADD | 2ffff |
| 3 | SENDMAIL | 3ddddd |
| 4 | ADD | 3ddddd |
| 5 | SENDMAIL | 2ffff |
| 6 | ADD | 1aaaaa |
| 7 | REMOVE | 3ddddd |
| 8 | ADD | 1ccccc |
| 9 | ADD | 3bbbbb |
| 10 | SENDMAIL | 3bbbbb |
+------+-----------+------------+
如您所见,表格购物车中有六个产品,表格操作中正好有六个 add
操作。此外,正如您所见,id=1 的用户购买了三种产品,但不是同时购买,因为有两个 session ; id=3 的用户也在不同时间购买了这两种产品等等...
SQL语句
SELECT u.user_id, c.session_id, c.cart_id, a.impression_id, a.action_name, u.email
FROM users AS u
INNER JOIN carts AS c ON c.user_id = u.user_id
INNER JOIN actions AS a ON a.session_id = c.session_id
ORDER BY u.user_id, c.session_id, c.cart_id
结果:
+---------+------------+---------+---------------+-------------+-----------------+
| user_id | session_id | cart_id | impression_id | action_name | email |
+---------+------------+---------+---------------+-------------+-----------------+
| 1 | 1aaaa | 1 | 1 | ADD | first@mail.org |
| 1 | 1aaaa | 1 | 6 | ADD | first@mail.org |
| 1 | 1aaaa | 4 | 1 | ADD | first@mail.org |
| 1 | 1aaaa | 4 | 6 | ADD | first@mail.org |
| 1 | 1cccc | 6 | 8 | ADD | first@mail.org |
| 2 | 2ffff | 2 | 5 | SENDMAIL | second@mail.org |
| 2 | 2ffff | 2 | 2 | ADD | second@mail.org |
| 3 | 3bbbb | 5 | 9 | ADD | third@mail.org |
| 3 | 3bbbb | 5 | 10 | SENDMAIL | third@mail.org |
| 3 | 3dddd | 3 | 3 | SENDMAIL | third@mail.org |
| 3 | 3dddd | 3 | 4 | ADD | third@mail.org |
| 3 | 3dddd | 3 | 7 | REMOVE | third@mail.org |
+---------+------------+---------+---------------+-------------+-----------------+
注意
:不保证 session 的唯一性。
(更新)工作 SQL Fiddle
更新:(查找和删除重复项)
我已经更新了 SQL Fiddle为了模拟重复记录(当用户在同一 session 中添加相同的产品时)。使用以下语句,您将能够检索那些重复的行。
SELECT c.card_id, c.user_id, c.product_id, c.session_id, a.action_name, a.impression_id
FROM cards As c
INNER JOIN actions AS a ON a.session_id = c.session_id
GROUP BY c.user_id, c.product_id, c.session_id, a.action_name
HAVING count(*) > 1
结果:
+---------+------------+------------+------------+-------------+-----------------+
| card_id | user_id | product_id | session_id | action_name | impression_id |
+---------+------------+------------+------------+-------------+-----------------+
| 1 | 1 | 5 | 1aaaa | ADD | 1 |
| 6 | 1 | 6 | 1cccc | ADD | 8 |
+---------+------------+------------+------------+-------------+-----------------+
在上述语句的 SELECT 部分中,您可以省略除 card_id 和 impression_id 之外的所有内容。在一个语句中删除这两个重复项有点棘手,因为您不能修改在同一查询的子查询中选择的同一个表。在这种情况下,我会避免棘手的部分(涉及另一个内部子查询),并会使用如下单独的语句删除重复项
-- delete duplicates from cards
--
DELETE FROM WHERE card_id IN (1,6)
-- delete duplicates from actions
--
DELETE FROM WHERE card_id IN (1,8)
更好的是,您可以检查用户是否已经添加了所选产品并且不要添加两次。
关于使用多列/键的 MySQL JOIN 3 表,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/24639504/