Question

我有一组以下表格：

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约束强制执行约束，即一个卖方只能拥有一个给定数量的发票。

我想创建一个新表，主要是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),
    ....
);

作为规范化工作，我想更改invoices以引用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

使用suppliers的复合主键：

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表的完整性，只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表中有更多与seller - buyer关系相关的列，我想这个设计会很有用。否则，您可以拥有一个从invoices表中收集数据的视图：

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

间接UNIQUE约束

1 个答案: