mysql - MySQL中REFERENCES关键字的使用

Question

我在 MySQL 中有以下表定义。它表示表Licenses 和License Server 之间的M:N 关系。

create table if not exists  served_license
(
    served_license_id       smallint            not null,
    version                 varchar(16)         not null,
    quantity                smallint            not null,
    expiry_date             char(16)            not null,
    license_server_id       tinyint             not null    references  license_server(license_server_id),
    license_id              varchar(64)         not null    references  license(feature, product),
    primary key (served_license_id)
);

这是关键字REFERENCES的正确用法吗？如果我将表定义更改为下面的定义，列的行为是否会有所不同？

create table if not exists  served_license
(
    served_license_id       smallint            not null,
    version                 varchar(16)         not null,
    quantity                smallint            not null,
    expiry_date             char(16)            not null,
    license_server_id       tinyint             not null,
    license_id              varchar(64)         not null,
    primary key (served_license_id),
    foreign key (license_server_id) references  license_server(license_server_id),
    foreign key (license_id) references license(feature, product)
);

Answer 1

当REFERENCES子句属于FOREIGN KEY规范时，MySQL仅处理它们。这是 manual 的引用:

MySQL parses but ignores “inline REFERENCES specifications” (as defined in the SQL standard) where the references are defined as part of the column specification. MySQL accepts REFERENCES clauses only when specified as part of a separate FOREIGN KEY specification.

因此，第一个版本不会强制执行外键关系，第二个版本会强制执行(假设您使用的是 InnoDB 存储引擎)。