这是什么使关系数据库的伟大：）

它将允许您搜索和分组比第二种方法更具体的标准。

说你想写一个查询，以便用户可以看到谁是他们的朋友。第二种方法要求你使用IN（），并且比使用JOINS慢得多。

I'm creating a table for allowing website users to become friends. I'm trying to determine which is the best table design to store and return a user's friends. The goal is to have fast queries and not use up a lot of db space.

I have two options:

Have individual rows for each friendship.

+----+-------------+-------------------+
| ID | User_ID     | Friend_ID         |
+----+-------------+-------------------+
| 1  | 102         | 213               |
| 2  | 64          | 23                |
| 3  | 4           | 344               |
| 4  | 102         | 2                 |
| 5  | 102         | 90                |
| 6  | 64          | 88                |
+----+-------------+-------------------+

Or store all friends in one row as CSV

    +----+-------------+-------------------+
    | ID | User_ID     | Friend_ID         |
    +----+-------------+-------------------+
    | 1  | 102         | 213,44,34,67,8    |
    | 2  | 64          | 23,33,45,105      |
    +----+-------------+-------------------+

When retrieving friends I can create an array using explode() however deleting a user would be trickier.

Edit: For second method I would separate each id in array in php for functions such as counting and others.

Which method do you think is better?

First method is definitely better. It's what makes relational databases great :)

It will allow you to search for and group by much more specific criteria than the 2nd method.

Say you wanted to write a query so users could see who had them as a friend. The 2nd method would require you to use IN() and would be much slower than simply using JOINS.