中级 SQL 表：有什么用？

Question

I'm managing to create my first complicated J2E Solution and in every tutorial I find some sort of intermediary tables usage, like here :

Tables : User, User_Roles, Roles While logic would simply add a key to user Table referring to it's role on Roles table, why the usage of that intermediary table ?

I thought it's one or two developpers choice, but everywhere I look for a tutorial, I find this sort of sql schema.

Is it better ? Does it help in something particular ? Speed, security ? Cause from a logic point of view, using one table User and a foreign key to Roles is better.

Thank you

Answer 1

This is a common database relationship modeling called MN (Many To Many). A User can have many Roles, and a Role can be assigned to many Users, so you need the intermediary table. Here's another example: a Teacher can teach many Classes, and each Class can be taught by many teachers (during different semesters, for example). In this case you need a Teacher-Class intermediary table.

A different kind of relationship is 1-N (one to N). A User can have many Telephones, but each Telephone is owned by a single User. In this case, a User's primary key (PK) is exported as a foreign key (FK) into the Telephones table. No need for an intermediary table.