如果我想从另一个表中获取 3 个不同的 Id（带有其他信息），是否需要在一个表中放置多个 Id？

Question

I am learning about SQL and tables, foreign keys, etc. To try it instead of only learning it I created 2 tables:

Table 1: Team :

Table 2: Karakter (that is "character" in Dutch)

A team has an Id ( TeamId ), UserId ( GebruikerId , this is not important for now), name ( Teamnaam ) and CharacterId ( KarakterId ).

I want a team to exist of 3 characters ( karakters ), and I know I can find character via foreign keys, but sins I want 3 characters and not only 1 I was wondering if in my table TeamI need to put KarakterId1, KarakterId2, KarakterId3?

Or do you usually do this in another simpler way that I don't know of (Because I can not find it)

Thanks in advance!

Answer 1

That's a many-to-many relationship between teams and characters: where one team has several characters, and a given character may belong to more than one team.

A normalized design implies creating a third table, called a bridge table, to represent that relationship, with one row for each team/character tuple, and foreign key constraints the reference the parent tables.

create table teams (
    team_id int primary key,
    user_id int not null,
    name varchar(50)
);

create table characters (
    character_id int primary key,
    name varchar(50)
);

create table team_characters (
    team_id int references teams(team_id),
    character_id int references characters(character_id),
    primary key (team_id, character_id)
);

Answer 2

Precisely verbalizing the constraint in natural language usually reveals the correct solution. Here are two possible examples, there can be more.

---

• Each karakter may be in more than one team; for each team: more than one karakter may be in that team.

karakter {K}
      PK {K}


team {T}
  PK {T}


team_karakter {T, K}
           PK {T, K}

          FK1 {K} REFERENCES karakter {K}
          FK2 {T} REFERENCES team     {T}

---

• Each karakter may be in more than one team; for each team: exactly three karakters are in that team.

karakter {K}
      PK {K}


team {T, K1, K2, K3}
  PK {T}

 FK1 {K1} REFERENCES karakter {K}
 FK2 {K2} REFERENCES karakter {K}
 FK3 {K3} REFERENCES karakter {K}

 CHECK ((K1 <> K2) AND (K1 <> K3) AND (K2 <> K3))

Note:

All attributes (columns) NOT NULL

PK = Primary Key
FK = Foreign Key