What other queries can I do with this many to many joining table?

问题

so..I am learning many to many relationship in mysql.

I created the following tables.

create table post(
    id INT NOT NULL PRIMARY KEY AUTO_INCREMENT,
    title varchar(40),
    content text,
    created int(11) default 0
);

create table category(
    id INT NOT NULL PRIMARY KEY AUTO_INCREMENT,
    name VARCHAR(35)
);

create table author(
    id int not null primary key auto_increment,
    name varchar(20)
);

create table postcategory(
    id int not null primary key auto_increment,
    post_id int,
    category_id int,
    foreign key(category_id) references category(id) 
    on delete no action,
    foreign key(post_id) references post(id)
    on delete no action
);

These are rows I Inserted

insert into author(name) values('James');
insert into author(name) values('Moon');
insert into author(name) values('Min');

insert into category(name) values('C#');
insert into category(name) values('Art');
insert into category(name) values('PHP');
insert into category(name) values('Programming');

insert into post(title) values ('Introduction to C#');
insert into post(title) values ('Modern Art');
insert into post(title) values ('PHP Classes');
insert into post(title) values ('Classes in C#');
insert into post(title) values ('Polymorphism in C#');
insert into post(title) values ('Classic Art');
insert into post(title) values ('OOP in PHP');



insert into postcategory(post_id,category_id) values(1,1);
insert into postcategory(post_id,category_id) values(4,1);
insert into postcategory(post_id,category_id) values(5,1);

insert into postcategory(post_id,category_id) values(2,2);
insert into postcategory(post_id,category_id) values(2,6);

insert into postcategory(post_id,category_id) values(3,3);
insert into postcategory(post_id,category_id) values(3,7);


insert into postcategory(post_id,category_id) values(1,4);
insert into postcategory(post_id,category_id) values(4,4);
insert into postcategory(post_id,category_id) values(5,4);

insert into postcategory(post_id,category_id) values(3,4);
insert into postcategory(post_id,category_id) values(7,4);

basically I have post, author, category, and a joining table named postcategory.

so far, I can think of two queries with postcategory table.

Query all the posts in C# category
Query all the categories with post id #1

Are there any other queries (I am sure there are..) that I can use with postcategory table?

I'm Just trying to learn many to many relationship usage cases.

回答1:

How about list the categories, and how many posts in each category, sorted from the category with the most posts to the category with the least.

回答2:

You can query if any posts belongs to more then one category.
E.g find posts which belong to C# and Programming.

来源：https://stackoverflow.com/questions/5100093/what-other-queries-can-i-do-with-this-many-to-many-joining-table

标签

mysql

many-to-many