I'm not really sure what feature you are referring to in MySQL.
A linking table is not so much a mySQL Feature as a common concept in Relational Database design.
the concept of a linking table is to use a third intermediary table to create a many to many relationship between to data tables.
In your case, you have users and categories, and you are trying to create a many to many relationship between them.
in other words, all users can relate to any of the available categories.
to do this you create a linking table.
the linking table creates the relationship be creating a record for each category the user should belong to.
I'll try to illustrate using sample data.
the table structure would be:
users:
userID - Primary Key Column
userName - text
etc
categories:
catID -Primary Key
catName -text
userCategories
usercatID -Primary key
usercatUserID relates to users.userID
usercatCatID - relates to categories.catID
table data:
users:
userID | userName | etc....
-------+------------+
1 | Tom |
-------+------------+
1 | John |
-------+------------+
1 | Adam |
-------+------------+
categories:
catID | catName |
------+-----------+
1 | Fruit |
------+-----------+
2 | Vegitable |
------+-----------+
3 | Meat |
------+-----------+
4 | seafood |
------+-----------+
Now, to assign Tom (userID = 1) to the Vegetable (catID = 2), Meat (catID = 3) and Seafood (catID = 4) Group, we would creat the folloing records in the userCategories table:
userCategories
usercatID | usercatUserID | usercatCatID |
----------+---------------+--------------+
1 | 1 | 2 |
----------+---------------+--------------+
2 | 1 | 3 |
----------+---------------+--------------+
3 | 1 | 4 |
----------+---------------+--------------+
to assign adam (userID = 3) to the Fruit (catID = 1), and vegitable (catID = 2) group:
userCategories
usercatID | usercatUserID | usercatCatID |
----------+---------------+--------------+
4 | 3 | 1 |
----------+---------------+--------------+
5 | 3 | 2 |
----------+---------------+--------------+
and to assign John (userID = 2) to the Meat (catID = 3), and Seafood (catID = 4 group:
userCategories
usercatID | usercatUserID | usercatCatID |
----------+---------------+--------------+
6 | 2 | 3 |
----------+---------------+--------------+
7 | 2 | 4 |
----------+---------------+--------------+
