同一张表上多对多关系的内部联接

Question

I came across a weird problem while I was trying to apply the Inner Join on the table which has 3 columns -

Table category_subcategory_relation

id (PK)
category_id_ref (FK)
subcategory_id_ref (FK)

The columns category_id_ref and subcategory_id_ref are the primary keys of the table category .

Table category

id (PK)
category_name (Varchar(200))

I want to Inner Join this two tables and get the columns as following -

Category_Name
Subcategory_Name

I am not able understand how would I be able to get the category_name column from the category table twice, once for the column Category_Name (FK) and again for the column Child_Category_Name (FK) .

Answer 1

You would have to perform two joins, one each on category_id_ref and subcategory_id_ref :

SELECT a.*, 
       b.category_name AS Category_Name, 
       c.category_name AS Subcategory_Name
FROM category_subcategory_relation a 
JOIN category b 
  ON a.category_id_ref    = b.id
JOIN category c 
  ON a.subcategory_id_ref = c.id

While your category table contains category and subcategory information, you'll have to look at them as two separate tables (one for category and other for subcategory ) to bring this information back to your category_subcategory_relation table.