Ruby on Rails: Join Tables Concept - ruby-on-rails

Question

So I have been out of the coding game for a while and recently decided to pick up rails. I have a question about the concept of Join tables in rails. Specifically:
1) why are these join tables needed in the database?
2) Why can't I just JOIN two tables on the fly like we do in SQL?

Answer 1

A join table allows a clean linking of association between two independent tables. Join tables reduce data duplication while making it easy to find relationships in your data later on.
E.g. if you compare a table called users:
| id | name |
-----------------
| 1 | Sara |
| 2 | John |
| 3 | Anthony |
with a table called languages:
| id| title |
----------------
| 1 | English |
| 2 | French |
| 3 | German |
| 4 | Spanish |
You can see that both truly exist as separate concepts from one another. Neither is subordinate to the other the way a single user may have many orders, (where each order row might store a unique foreign_key representing the user_id of the user that made it).
When a language can have many users, and a user can have many languages -- we need a way to join them.
We can do that by creating a join table, such as user_languages, to store every link between a user and the language(s) that they may speak. With each row containing every matchup between the pairs:
| id | user_id | language_id |
------------------------------
| 1 | 1 | 1 |
| 2 | 1 | 2 |
| 3 | 1 | 4 |
| 4 | 2 | 1 |
| 5 | 3 | 1 |
With this data we can see that Sara (user_id: 1) is trilingual, while John(user_id: 2) and Anthony(user_id: 3) only speak English.
By creating a join table in-between both tables to store the linkage, we preserve our ability to make powerful queries in relation to data on other tables. For example, with a join table separating users and languages it would now be easy to find every User that speaks English or Spanish or both.
But where join tables get even more powerful is when you add new tables. If in the future we wanted to link languages to a new table called schools, we could simply create a new join table called school_languages. Even better, we can add this join table without needing to make any changes to the languages SQL table itself.
As Rails models, the data relationship between these tables would look like this:
User --> user_languages <-- Language --> school_languages <-- School
By default every school and user would be linked to Language using the same language_id(s)
This is powerful. Because with two join tables (user_languages & school_languages) now referencing the same unique language_id, it will now be easy to write queries about how either relates. For example we could find all schools who speak the language(s) of a user, or find all users who speak the language(s) of a school. As our tables expand, we can ride the joins to find relations about pretty much anything in our data.
tl;dr: Join tables preserve relations between separate concepts, making it easy to make powerful relational queries as you add new tables.