Foreign key for many to many relationship in sql - sql

Question

I have an SQL table called Listing which is representing houses that have been rented. The table has a primary key id and another field called amenities with the things each house had to offer. The amenities of each house are separated from each other with a comma. For example TV, Internet, Bathroom.
I used the following commands to create a table called Amenity with all the unique different amenities offered and a SERIAL number for each amenity.
CREATE TABLE Amenity AS(
SELECT DISTINCT regexp_split_to_table(amenities,',') FROM Listing
);
ALTER TABLE Amenity
RENAME regexp_split_to_table to amenity_name;
ALTER TABLE Amenity ADD COLUMN amenity_id SERIAL;
ALTER TABLE Amenity ADD PRIMARY KEY(amenity_id);
My problem is that I need to connect these two tables with a foreign key and I don't know how since the relationship between them is a many to many relationship. I have checked other questions regarding foreign keys in many to many relations but could not find anything similar. If there exists something similar please explain the way it is similar to my question.

Answer 1

You must create another table which will hold the one-to-many relationships between a house and its amenities.
So your 3 tables looks look like this:
Table HOUSE
+----------+------------+
| house_id | house_name |
+----------+------------+
| 1 | Uncle Bob |
+----------+------------+
| 2 | Mom Sara |
+----------+------------+
Table AMENITIES
+------------+--------------+
| amenity_id | amenity_name |
+------------+--------------+
| 1 | TV |
+------------+--------------+
| 2 | Internet |
+------------+--------------+
| 3 | Kitchen |
+------------+--------------+
Table HOUSE_AMENITIES
+----------+------------+
| house_id | amenity_id |
+----------+------------+
| 1 | 1 |
+----------+------------+
| 2 | 1 |
+----------+------------+
| 2 | 2 |
+----------+------------+
| 2 | 3 |
+----------+------------+
So the house Uncle Bob has only TV while the house Mom Sara has TV, Internet and a fully equipped kitchen.
Remember - you should never use the same column to store multiple values (separated with comma). In all such cases you have to use another table, converting the multiple comma-separated values into distinct rows inside this detail table and referencing the primary key of the master table.