简单的MYSQL数据查询？

Question

Just a really simple question. In my SQL database I have a column named "friend_count" in table "users" to record all friends the logged in user has. This works with UserID numbers and I am trying to figure out how to record them all. Here's my example:

USER #29 becomes friends with USER #422

In the user database under USER #29's info in the column "friend_count" there will then be the number "422", if he then becomes friends with USER #500, it will show "422, 500" and so on... If he deletes one, that particular number is removed from the box. The script will then search these numbers through the user database to show a list of the people you are friends with.

How can I do this? Any ideas? Thanks!

Answer 1

Any time you have a delimited list of values in a column it's almost always a sign that the data model is incorrect. (Actually, I can't think of an exception to this, but I'll stick with "almost always" just to be safe.)

In this case you have two types of entities:

• User
• Friendship

A friendship, though not a physical object, is a conceptual entity in and of itself. It connects two users and can add more information related to the friendship itself but not necessarily to the two users.

So you might have tables like this:

User
--------
ID
Name
etc.

Friendship
--------
ID
OriginatingUser
AcceptingUser
BecameFriendsOn
etc.

So OriginatingUser might be the user who sent the friend request, and AcceptingUser might be the user who accepted it. BecameFriendsOn is the date it was accepted. You'd probably want to have statuses and other dates to keep track of pending requests, denied requests, etc. But all of this information is related to the friendship, not necessarily to the users.

The concept you're looking to understand here is called a Foreign Key. The OriginatingUser and AcceptingUser columns are the same data type as the ID column on the User table. You would create the Friendship table such that those columns are foreign keys to the User table, this enforces the integrity of the data so that you can't create a Friendship record without two valid and existing User records.

Then to get the list of friends, you'd join the tables in a query. Perhaps something like this:

SELECT
  User.Name
FROM
  Friendship
  INNER JOIN User ON Friendship.AcceptingUser = User.ID
WHERE
  Friendship.OriginatingUser = ?

When supplied with the ID of the originating user, this would get all of the names of users to whom that user sent a friend request. You can further build on the query to also get users who sent this same user a friend request, and so on. By making use of the key foreign key relationships between tables which represent different types of entities in the system, you can construct very robust queries to view that data in lots of different ways.

Answer 2

You are describing a relationship between two entities so you can create a table to store the details of that relationship.

Suppose that you have your user table with a userid column and other columns. You can then create a friends table with two columns that are both foreign keys to the user table.

      friends = (user, friend)

Thus, for each friend that user #29 gets you need to add a row into the friends table. For example:

USER
    ID   NAME  ...
    29   Sam
    30   Henry
    32   Jane


    Friends
    user     friend
    29       30
    29       32

Sam is friends with both Jane and Henry but Jane and Henry are not friends.

Answer 3

我将创建一个带有两个列“用户”和“朋友”的表“友谊”，然后开始添加用户/朋友ID对

Answer 4

users table: id, username, whatever else

friends table: relationship_id, user_id, user_friend_id

example query to get a list of IDs that belong to the users friends:

SELECT f.user_friend_id FROM users u 
LEFT JOIN friends f ON f.user_id = u.id
WHERE u.id = {$user_id}

Answer 5

A very simple approach, assuming that if UserA becomes friends with UserB , then UserB also becomes friends with UserA .

Usually, Comma Separated Lists are not recommended, as they will become a pain when the list is very large. A simpler approach will be, make a table friends with columns user_id and friend_id Where user_id and friend_id are the respective UserIDs .

Now when you want to add a friend to someone's list, use the following :

INSERT INTO users (user_id,friend_id) VALUES(UserA,UserB),(UserB,UserA)

Now when you execute the above query, you will have 2 new rows in your friends table :

user_id friend_id

UserA UserB UserB UserA

When you want to get the list of a user's friends, use :

SELECT friend_id FROM friends WHERE user_id=(Your user's ID)

This will return a row one by one, and hence will give you all the IDs of friends of a particular user.

Now when you want to delete a friend, use :

DELETE FROM friends WHERE (user_id,friend_id) IN ((user_id,friend_id),(friend_id,user_id))

This removes both the rows from the table, which means that the relationship between UserA and UserB is deleted, and neither of them is friend of each other.