是否存在SQL表约束以确保两列之间存在关系？

我正在努力解决以下问题

CREATE TABLE leagues (
    id SERIAL PRIMARY KEY,
    name TEXT,
    ...
);

CREATE TABLE players (
    id SERIAL PRIMARY KEY,
    first_name TEXT,
    last_name TEXT,
    ...
);

CREATE TABLE league_members (
    league_id INTEGER REFERENCES leagues (id),
    player_id INTEGER REFERENCES players (id),
    PRIMARY KEY (league_id, player_id)
);

CREATE TABLE games (
    id SERIAL PRIMARY KEY,
    league_id INTEGER REFERENCES leagues (id),
    winner INTEGER REFERENCES players (id),
    loser INTEGER REFERENCES players (id)
);

在游戏表中，我是否可以使用约束来检查赢家和输家是否都是同一联盟的成员

例如：

玩家表：

---- id ---- first_name ----
     1       "John"
     2       "Joe"
     3       "Jill"

---- id ---- league_id ---- player_id ----
     1           1              1
     2           1              2
     3           2              3

SELECT count(*) FROM league_members WHERE league_id = 1 AND (player_id = 1 OR player_id = 3);

联赛表：

---- id ---- name ----
     1       "League 1"
     2       "League 2"

联盟成员表：

---- id ---- first_name ----
     1       "John"
     2       "Joe"
     3       "Jill"

---- id ---- league_id ---- player_id ----
     1           1              1
     2           1              2
     3           2              3

SELECT count(*) FROM league_members WHERE league_id = 1 AND (player_id = 1 OR player_id = 3);

我希望确保在游戏表中，以下内容会导致错误：

INSERT INTO games (league_id, winner, loser) VALUES (1,1,3);

这是不允许的，因为球员3不在一级联赛。现在我只是在league_members表上运行以下查询：

---- id ---- first_name ----
     1       "John"
     2       "Joe"
     3       "Jill"

---- id ---- league_id ---- player_id ----
     1           1              1
     2           1              2
     3           2              3

SELECT count(*) FROM league_members WHERE league_id = 1 AND (player_id = 1 OR player_id = 3);

他们确保计数为2，这意味着两名球员都是联盟中id为1的成员。如果计数为2，则我允许插入到games表中，否则不允许。有没有办法做到这一点，但让数据库处理约束，这样我就不必在尝试插入到games表之前进行额外的查询

---

谢谢

几个外键应该可以让事情保持正常：

CREATE TABLE games (
    id SERIAL PRIMARY KEY,
    league_id INTEGER REFERENCES leagues (id),
    winner INTEGER REFERENCES players (id),
    loser INTEGER REFERENCES players (id),
    constraint FK_League_Winner FOREIGN KEY (league_id,winner)
       REFERENCES league_members (league_id,player_id),
    constraint FK_League_Loser FOREIGN KEY (league_id,loser)
       REFERENCES league_members (league_id,player_id)
);

---

由于这两个键都共享此表中的

league\u id

列，因此不可能出现成员来自不同联盟的情况。

两个外键应该可以使事情保持正常：

CREATE TABLE games (
    id SERIAL PRIMARY KEY,
    league_id INTEGER REFERENCES leagues (id),
    winner INTEGER REFERENCES players (id),
    loser INTEGER REFERENCES players (id),
    constraint FK_League_Winner FOREIGN KEY (league_id,winner)
       REFERENCES league_members (league_id,player_id),
    constraint FK_League_Loser FOREIGN KEY (league_id,loser)
       REFERENCES league_members (league_id,player_id)
);

---

由于这两个键都共享此表中的

league\u id

列，因此不可能存在成员来自不同联盟的情况。

！谢谢！令人惊叹的！谢谢！