如何设计具有多对多关系的表?

For example, A Users entity has a friends property, how can I design this friends property, in my thought there are 2 ways:

例如，一个用户实体有一个朋友属性，我如何设计这个朋友属性，在我看来有两种方式:

friends property is a String with all usernames splitted by "," in this way it's hard to read and modify.
friends属性是一个字符串，它的所有用户名都被“，”分割，因此很难读取和修改。
friends property is a Set like Set<Users>, but in this way I don't know how to write in entities?
朋友属性是一个集 <用户> ，但这样我不知道如何在实体中写?

Anyone knows the best practise?

有人知道最好的练习吗?

2 个解决方案

#1

If a User can have multiple friends you could annotate your User entity like this:

如果一个用户可以有多个朋友，您可以这样注释您的用户实体:

@Entity
public class User
{
    @Id
    private Long id;
    private String name;
    @ManyToMany
    @JoinTable(
            name = "user_friends",
            joinColumns =
            { @JoinColumn(
                    name = "user_id") },
            inverseJoinColumns =
            { @JoinColumn(
                    name = "friend_id") })
    private Set<User> friends;
}

This way a table will get created for User and a join table for the relationship between Users. The User table will have 2 columns, 'id' and 'name'. The user_friend table will have 2 columns, 'user_id' and 'friend_id'. The columns in user_friend are both foreign keys to the User table.

这样，将为用户创建一个表，为用户之间的关系创建一个连接表。用户表将有两列，“id”和“name”。user_friend表将有两个列“user_id”和“friend_id”。user_friend中的列都是用户表的外键。

#2

This is covered Enterprise Model Patterns by Hay.

这是由Hay覆盖的企业模型模式。

A party represents a person (or an organization):

一方代表一个人(或组织):

Party
id
name

A party can have a relationship to another party, over a time period:

一方可以与另一方有一段时间的关系:

PartyRelationship
fromPartyId
toPartyId
fromDate
toDate nullable

A basic diagram:

一个基本的图:

Party -< PartyRelationship >- Party

Sample SQL:

示例SQL:

insert into party values (1, 'Jerry');
insert into party values (2, 'Neil');

insert into partyRelationship values (1, 2, getDate(), null);

#1