Handling many-to-many relationship in a sql query - sql

Question

So as you know in data modeling, the many-to-many relationships are handled by creating a bridge table. This will enable us to have foreign key constraints.
My question relating to query data from tables that have many-to-many relationships.
I will give an example. Let's say we have the following tables
Table 1
Column 1
Column 2
Column 3
Table 2
Column 1
Column 4
Column 5
Table 3 (bridge table)
Column 1
Ok, so when I tried to query data from table 1 and left joining with table 2 I got the same results as query data from table left joining with table 3 and left joining with table 2. And that makes me wonder if the bridge table is a necessary step to include in a SQL query!
Thank you in advance :)

Answer 1

Your bridge table has just one column, which suggests that you are confusing one-to-many relationships (or many one-to-one), and many-to-many relationships.
In a one-to-many relationship, the child table refers directly to the parent table, and there is no need for a bridge table:
create table orders (
order_id int primary key,
order_date date,
...
);
create table order_items (
order_item_id int primary key,
order_id int references orders(order_id),
...
)
In a many-to-many relationship, the bridge table has (at least) two columns, one for each of the referential table that come into play in the relation:
create table authors (
author_id int primary key,
name varchar(50),
...
);
create table books (
book_id int primary key,
title varchar(50),
...
);
create table book_authors (
author_id int references authors(author_id),
book_id int references books(book_id),
primary key (author_id, book_id)
)

Answer 2

Let's make your example less abstract. There are posts with dislikes and likes. Table 1 = likes, Table 2 = dislikes.
Table likes (post_id, who, why)
Table dislikes (post_id, who, why)
The bridge table is then
Table posts (post_id, content)
This is an example where two child tables are related by a parent table. You don't create the bridge table in order to get an m:n relation between the two child tables, but because the child tables make no sense without their parent table. The child tables are m:n related, but we don't usually call this an m:n relation, because the tables are only losely related (a like belongs to a post and a dislike belongs to a post, but the like doesn't really belong to the dislike).
When talking about m:n relations, we are usally not talking about parent/child relationship.
An example: An order can contain many products and a product can be in many orders.
Table orders (order_id, date)
Table products (product_id, name, price)
The bridge table is the position in the order:
Table order_detail (order_id, product_id, amount)
There is no parent/child relation between orders and products, but the two are related, because products get ordered. This is what we typically call an m:n relation. The bridge table establishes the relation between order and product.