sqlmodel/docs/tutorial/many-to-many/index.md

# Many to Many - Intro

We saw how to work with <abbr title="Also called Many-to-One">One-to-Many</abbr> relationships in the data.

But how do you handle **Many-to-Many** relationships?

Let's explore them. 🚀

## Starting from One-to-Many

Let's start with the familiar and simpler option of **One-to-Many**.

We have one table with teams and one with heroes, and for each **one** team, we can have **many** heroes.

As each team could have multiple heroes, we wouldn't be able to put the Hero IDs in columns for all of them in the `team` table.

But as each hero can belong **only to one** team, we have a **single column** in the heroes table to point to the specific team (to a specific row in the `team` table).

The `team` table looks like this:

<table>
<tr>
<th>id</th><th>name</th><th>headquarters</th>
</tr>
<tr>
<td>1</td><td>Preventers</td><td>Sharp Tower</td>
</tr>
<tr>
<td>2</td><td>Z-Force</td><td>Sister Margaret’s Bar</td>
</tr>
</table>

!!! tip
    Notice that it doesn't have any foreign key to other tables.

And the `hero` table looks like this:

<table>
<tr>
<th>id</th><th>name</th><th>secret_name</th><th>age</th><th>team_id</th>
</tr>
<tr>
<td>1</td><td>Deadpond</td><td>Dive Wilson</td><td>null</td><td>2</td>
</tr>
<tr>
<td>2</td><td>Spider-Boy</td><td>Pedro Parqueador</td><td>null</td><td>1</td>
</tr>
<tr>
<td>3</td><td>Rusty-Man</td><td>Tommy Sharp</td><td>48</td><td>1</td>
</tr>
</table>

We have a column in the `hero` table for the `team_id` that points to the ID of a specific team in the `team` table.

This is how we connect each `hero` with a `team`:

<img alt="table relationships" src="/img/databases/relationships.svg">

Notice that each hero can only have **one** connection. But each team can receive **many** connections. In particular, the team **Preventers** has two heroes.

## Introduce Many-to-Many

But let's say that as **Deadpond** is a great chracter, they recruit him to the new **Preventers** team, but he's still part of the **Z-Force** team too.

So, now, we need to be able to have a hero that is connected to **many** teams. And then, each team, should still be able to receive **many** heroes. So we need a **Many-to-Many** relationship.

A naive approach that wouldn't work very well is to add more columns to the `hero` table. Imagine we add two extra columns. Now we could connect a single `hero` to 3 teams in total, but not more. So we haven't really solved the problem of supporting **many** teams, only a very limited fixed number of teams.

We can do better! 🤓

## Link Table

We can create another table that would represent the link between the `hero` and `team` tables.

All this table contains is two columns, `hero_id` and `team_id`.

Both columns are **foreign keys** pointing to the ID of a specific row in the `hero` and `team` tables.

As this will represent the **hero-team-link**, let's call the table `heroteamlink`.

It would look like this:

<img alt="many-to-many table relationships" src="/img/tutorial/many-to-many/many-to-many.svg">

Notice that now the table `hero` **doesn't have a `team_id`** column anymore, it is replaced by this link table.

And the `team` table, just as before, doesn't have any foreign key either.

Specifically, the new link table `heroteamlink` would be:

<table>
<tr>
<th>hero_id</th><th>team_id</th>
</tr>
<tr>
<td>1</td><td>1</td>
</tr>
<tr>
<td>1</td><td>2</td>
</tr>
<tr>
<td>2</td><td>1</td>
</tr>
<tr>
<td>3</td><td>1</td>
</tr>
</table>

!!! info
    Other names used for this **link table** are:

    * association table
    * secondary table
    * junction table
    * intermediate table
    * join table
    * through table
    * relationship table
    * connection table

    I'm using the term "link table" because it's short, doesn't collide with other terms already used (e.g. "relationship"), it's easy to remember how to write it, etc.

## Link Primary Key

Cool, we have a link table with **just two columns**. But remember that SQL databases [require each row to have a **primary key**](../../databases.md#identifications-primary-key){.internal-link target=_blank} that **uniquely identifies** the row in that table?

Now, what is the **primary key** in this table?

How to we identify each unique row?

Should we add another column just to be the **primary key** of this link table? Nope! We don't have to do that. 👌

**Both columns are the primary key** of each row in this table (and each row just has those two columns). ✨

A primary key is a way to **uniquely identify** a particular row in a **single table**. But it doesn't have to be a single column.

A primary key can be a group of the columns in a table, which combined are unique in this table.

Check the table above again, see that **each row has a unique combination** of `hero_id` and `team_id`?

We cannot have duplicated primary keys, which means that we cannot have duplicated links between `hero` and `team`, exactly what we want!

For example, the database will now prevent an error like this, with a duplicated row:

<table>
<tr>
<th>hero_id</th><th>team_id</th>
</tr>
<tr>
<td>1</td><td>1</td>
</tr>
<tr>
<td>1</td><td>2</td>
</tr>
<tr>
<td>2</td><td>1</td>
</tr>
<tr>
<td>3</td><td>1</td>
</tr>
<tr>
<td>3 🚨</td><td>1 🚨</td>
</tr>
</table>

It wouldn't make sense to have a hero be part of the **same team twice**, right?

Now, just by using the two columns as the primary keys of this table, SQL will take care of **preventing us from duplicating** a link between `hero` and `team`. ✅

## Recap

An intro with a recap! That's weird... but anyway. 🤷

Now you have the theory about the **many-to-many** relationships, and how to solve them with tables in SQL. 🤓

Now let's check how to write the SQL and the code to work with them. 🚀