MySQL self connection query "suggestions collection"

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Self connect query

Imagine the following scenario ： An e-commerce website wants to make hierarchical classification of products in the station , There are several subclasses under a category , There will also be other subclasses under subclasses . For example, there are notebooks under the category of digital products , Desktop computer , Smart phones and so on ; The notebook , Desktop computer , Smart phones can be classified by brand ; Brands can be classified according to price , wait . Perhaps these categories will reach a very deep level , Present a tree like structure . So how can these data be represented in the database ？ We can create two fields in the database to store id And category name , Use the third field to store the contents of the subclass or parent of the category id, Finally, query the desired results through self connection .

Self join query is actually equivalent to join query , I need two tables , Just its left watch （ Parent table ） And the right watch （ Sub table ） They are all themselves . When doing self connection query , It's about connecting with yourself , Give the parent table and the child table two different aliases , Then attach the connection conditions . See the following example ：

1. Create data table ：

create table tdb_cates(
	id smallint primary key auto_increment,
	cate_name varchar(20) not null,
	parent_id smallint not null
);

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notes ：cate_name Indicates the name of the classification ,parent_id Represents the parent class id.

2. insert data ：

insert into tdb_cates(cate_name, parent_id) values(' Digital products ', 0);
insert into tdb_cates(cate_name, parent_id) values(' Household products ', 0);
insert into tdb_cates(cate_name, parent_id) values(' The notebook ', 1);
insert into tdb_cates(cate_name, parent_id) values(' A smart phone ', 1);
insert into tdb_cates(cate_name, parent_id) values(' Electrical appliances ', 2);
insert into tdb_cates(cate_name, parent_id) values(' furniture ', 2);
insert into tdb_cates(cate_name, parent_id) values(' The refrigerator ', 5);
insert into tdb_cates(cate_name, parent_id) values(' Washing machine ', 5);
insert into tdb_cates(cate_name, parent_id) values(' Car brand ', 0);
insert into tdb_cates(cate_name, parent_id) values(' buick ', 9);
insert into tdb_cates(cate_name, parent_id) values(' BMW ', 9);
insert into tdb_cates(cate_name, parent_id) values(' Chevrolet ', 9);
insert into tdb_cates(cate_name, parent_id) values(' Home textiles ', 0);

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Query results ：

3. Query all categories and their parent classes ： Suppose there are two tables on the left and right （ All are tdb_cates）, The left table is a child table , The right table is the parent table ; Query sub table id, Of child tables cate_name, The parent table of the cate_name; The join condition is a child table parent_id Equal to of parent table id.

select s.id, s.cate_name, p.cate_name from tdb_cates s left join tdb_cates p on s.parent_id=p.id;

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Query results ：

4. Query all categories and their subclasses ： Or suppose there are two tables （ All are tdb_cates）, The left table is a child table , The right table is the parent table ; Query sub table id, Of child tables cate_name, The parent table of the cate_name; The join condition is a child table id Equal to of parent table parent_id.

select s.id, s.cate_name, p.cate_name from tdb_cates s left join tdb_cates p on p.parent_id=s.id;

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Query results ：

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