MySQL中的一对多关系 - 如何构建模型？

Question

I have got two tables:

1) Area 2) Map

Each Area shall have at least 1 Map, but can also have more than one Map.

One Map can only belong to one Area.

How to build this in MySQL?

Answer 1

create table Area(id int primary key auto_increment, name varchar(100));

create table Map(id int primary key auto_increment, 
                 area_id int not null,
                 name varchar(100),
                 foreign key (area_id) references area(id));

SqlFiddle

Each Map MUST have an Area , as area_id is not null (and is a Foreign key on Area )

But you won't be able (and it's not desired) to have "at least one map" for each area.

One day, you'll have to create an Area . And it won't have any Map at this time. Or make "regular" checks to see the Areas without any Map.

You may want to delete an Area , if it has no more related Map , when you delete a Map .

Answer 2

Add a Foreign key in Map that references the Area's Primary Key. That will enforce a one-to-many relationship between Maps and Areas.

As for enforcing a minimum of one map per area (if that is necessary) there are some ideas in this post here . One of the simpler solutions would be to create a view that only displays areas which have maps:

CREATE VIEW viewAreas AS
SELECT * 
FROM Areas, Maps
WHERE Areas.ID = Maps.AreaID;

This way, you can create an area, and then add maps to it. You can also enforce the Foreign Key in maps to be NOT NULL, so a map must always have an area.

Answer 3

Map和Area各有一个表，Map上的外键链接到Area。