sql - 间接 UNIQUE 约束

Question

我有以下一组表格:

CREATE TABLE sellers (
    id       integer PRIMARY KEY,
    ....
);

CREATE TABLE buyers (
    id       integer PRIMARY KEY,
    ....
);

CREATE TABLE invoices (
    id        integer PRIMARY KEY,
    number    varchar(40),
    buyer_id  integer REFERENCES buyers(id),
    seller_id integer REFERENCES sellers(id),
    UNIQUE(seller_id, number),
    ....
);

UNIQUE 约束强制一个卖家只能有一张具有给定编号的发票。

我想创建一个新表，主要作为 buyers 和 sellers 之间的多对多关系:

CREATE TABLE suppliers (
    id        integer PRIMARY KEY,
    buyer_id  integer REFERENCES buyers(id),
    seller_id integer REFERENCES sellers(id),
    ....
);

作为标准化工作，我想将 invoices 更改为引用 suppliers，如下所示:

CREATE TABLE invoices (
    id        integer PRIMARY KEY,
    number    varchar(40),
    supplier_id  integer REFERENCES suppliers(id),
    ....
);

我的问题是:如何替换 UNIQUE 约束？

Answer 1

为供应商使用复合主键:

CREATE TABLE suppliers (
    buyer_id  integer REFERENCES buyers(id),
    seller_id integer REFERENCES sellers(id),
    ...
    PRIMARY KEY (buyer_id, seller_id),
    ...
);

并保持 invoices 表完整，只是将两个外键更改为一个:

CREATE TABLE invoices (
    id        integer PRIMARY KEY,
    number    varchar(40),
    buyer_id  integer,
    seller_id integer,
    UNIQUE (seller_id, number),
    FOREIGN KEY (buyer_id, seller_id)
      REFERENCES suppliers (buyer_id, seller_id),
    ....
);

如果您计划在 suppliers 表中包含更多与 seller - buyer 关系相关的列，我想这种设计会很有用。否则，您可以拥有一个从 invoices 表收集数据的 View :

CREATE VIEW suppliers AS
SELECT DISTINCT buyer_id, seller_id
FROM invoices ;