Mysql Query查找朋友和下相同订单的朋友的朋友

Question

I have 3 tables as below

User

- id integer primary_key
- user_name 

Friends

- id autoincrement primary_key 
- user1 integer foreign_key(User.id)
- user2 integer foreign_key(User.id)

Orders

- id integer primary_key autoincrement
- product_id integer 
- by_user foreign_key(User.id) 

I have been provided with user's id and product_id .

I have to write a query which will give me all the friends of this user who placed order for same product along with all friends of friends of this user who placed order for same products .

How i can write such a query in mysql ?

---

Explanation with sample data

User

id    |   name
------------------
1     |   peter   
2     |   sam
3     |   xang
4     |   aisha
5     |   rita
6     |   mojo
7     |   raj
8     |   ben

Friends

(if user with id 1 and 2 are friends then it can be stored as user1 = 1 and user2 = 2 OR user1 = 2 and user2 = 1 , both means a relationship between users)

id          user1        user2 
---------------------------------------------
1      |     1       |     2      
2      |     2       |     3      
3      |     3       |     4      
4      |     4       |     2      
5      |     5       |     6      
6      |     6       |     8      
7      |     8       |     1   

Orders

id         product_id           by_user 
-----------------------------------------------
1      |    100        |          2        
2      |    100        |          3       
3      |    100        |          4       
4      |    100        |          5       
5      |    100        |          7       
6      |    101        |          5       
7      |    102        |          6       
8      |    103        |          1          

So Provided data is :

User = 1 and product = 100

What we have to do :

friends of user with id = 1 are [2,8] (lets call it f1)

friends of users in f1 are = [1,3,4,6] (lets call it f2)

Now give userid from order table who placed order for product_id = 100 and have user id in f1+f2 ie [2,8,1,3,4,6]

So here result should be :

[2,3,4]

Answer 1

SELECT DISTINCT u.id, u.name
FROM
    Friends f1
    INNER JOIN Friends f2
    ON f1.user1 = f2.user1
    OR f1.user1 = f2.user2
    OR f1.user2 = f2.user1
    OR f1.user2 = f2.user2
    INNER JOIN Orders o
    ON (f1.user1 = o.by_user
    OR f1.user2 = o.by_user
    OR f2.user1 = o.by_user
    OR f2.user2 = o.by_user)
    AND o.product_id = 100
    INNER JOIN Users u
    ON o.by_user = u.id
WHERE   
    (f1.user1 = 1
    OR f1.user2 = 1)

This answer is updated to account for Friends of Friends and to add an explanation for you.

• Query the Friends table for any record that contains user 1 which will give you User 1 as well as their immediate friends.
• Add a SELF JOIN to Friends table with all of the combinations of user1 = user1, 2 to 2 1 to 2 2 to 1 as OR to get all of the Fiends of Friends
• Get all orders related to any combination of the 4 user columns you now have and limit the results by product_id = 100
• You now have duplicate results so select DISTINCT to get 1 record per user.

Answer 2

This should be pretty self-explanatory, but just to make it even more clear: Create views to tackle the problem one step at a time, and use the UNION keyword to join different degrees of friendship.

DROP TABLE IF EXISTS Users;
CREATE TABLE Users (
    id INTEGER PRIMARY KEY,
    name NVARCHAR(50)
);

DROP TABLE IF EXISTS Friends;
CREATE TABLE Friends (
  user1 INTEGER,
  user2 INTEGER,
  FOREIGN KEY (user1) REFERENCES Users(id),
  FOREIGN KEY (user2) REFERENCES Users(id)
  );

DROP TABLE IF EXISTS Orders;
CREATE TABLE Orders (
  id INTEGER PRIMARY KEY,
  by_user INTEGER,
  FOREIGN KEY (by_user) REFERENCES Users(id) 
);

INSERT INTO Users (id,name) VALUES (1,"a"),(2,"b"),(3,"c"),(4,"d"),(5,"e"),(6,"f");

INSERT INTO Friends (user1,user2) VALUES (1,2),(2,1),(2,3),(3,2);

INSERT INTO Orders (id,by_user) VALUES(1,1);

DROP VIEW IF EXISTS FirstOrderFriends;
CREATE VIEW FirstOrderFriends AS
    SELECT DISTINCT Users.id AS id, f1.user2 AS friend1
        FROM Users 
            LEFT JOIN Friends AS f1 ON f1.user1 = Users.id;

DROP VIEW IF EXISTS SecondOrderFriends;
CREATE VIEW SecondOrderFriends AS
    SELECT DISTINCT Users.id AS id, f2.user2 AS friend2
        FROM Users 
            LEFT JOIN Friends AS f1 ON f1.user1 = Users.id  
            LEFT JOIN Friends As f2 ON f2.user1 = f1.user2;

DROP VIEW IF EXISTS FirstAndSecondOrderFriends;
CREATE VIEW FirstAndSecondOrderFriends AS
    SELECT DISTINCT * FROM (
            SELECT * FROM FirstOrderFriends
        UNION
            SELECT * FROM SecondOrderFriends
    );

SELECT * 
    FROM Users LEFT JOIN FirstAndSecondOrderFriends 
        ON Users.id = FirstAndSecondOrderFriends.id
    WHERE Users.id = 1;

I am just using WHERE Users.id = 1; joining the ORDERS table is left as an exercise to the student.