Question

这是我尝试创建表时来自show engine innodb status;的错误消息：

------------------------
LATEST FOREIGN KEY ERROR
------------------------
110628 16:56:07 Error in foreign key constraint of table test/menu_items:
foreign key(id_menu)
                references menus(id)
                on update cascade
                on delete cascade,
        foreign key(id_item)
                references items(id)
                on update cascade
                on delete cascade,
        primary key(id_menu, id_item)
) engine=InnoDB:
Cannot find an index in the referenced table where the
referenced columns appear as the first columns, or column types
in the table and the referenced table do not match for constraint.
Note that the internal storage type of ENUM and SET changed in
tables created with >= InnoDB-4.1.12, and such columns in old tables
cannot be referenced by such columns in new tables.
See http://dev.mysql.com/doc/refman/5.1/en/innodb-foreign-key-constraints.html
for correct foreign key definition.

这些是相关的SQL语句：

create table if not exists menus (                                                                                            
    id              mediumint unsigned not null auto_increment,
    id_restaurant   mediumint unsigned not null,
    name            varchar(50) not null,
    description     varchar(255) default null,
    foreign key(id_restaurant)
        references restaurants(id)
        on update cascade
        on delete cascade,
    primary key(id)
) engine=InnoDB;

create table if not exists items (
    id              mediumint unsigned not null auto_increment,
    id_restaurant   mediumint unsigned not null,
    name            varchar(50),
    description     text,
    type            enum('appetizer','salad','soup','entree','dessert','drink','other'),
    price           decimal(4,2),
    foreign key(id_restaurant)
        references restaurants(id)
        on update cascade
        on delete cascade,
    primary key(id)
) engine=InnoDB;

create table if not exists order_items (
    id_order        bigint unsigned not null,
    id_item         mediumint unsigned not null,
    item_name       varchar(50),
    item_description    text,
    item_price      decimal(4,2),
    notes           varchar(1024),
    quantity        smallint unsigned,
    foreign key(id_order)
        references orders(id)
        on update cascade
        on delete cascade,
    foreign key(id_item)
        references items(id)
        on update cascade
        on delete cascade,
    primary key(id_order, id_item)
) engine=InnoDB;

删除menu_items.id_menu和相应的外键/主键可以正确解析SQL语句。

为什么我不能从menu_items中对菜单（id）进行外键引用？

Answer 1

您的数据类型不匹配：

create table order_items (
    id_order bigint unsigned not null, -- BIGINT

create table items (
    id mediumint unsigned not null auto_increment, -- MEDIUMINT

但order_items.id_order引用items.id，因此它们应该相同。

尝试将order_items id_order更改为medium int：

order_items (
    id_order        mediumint unsigned not null,

无法弄清楚外键约束语句有什么问题

1 个答案: