Question

let say i have two table, User and Language

user (Table Name)
-----------------
user_id (PK)
user_name
mobile
...
....

language (Table Name)
---------------------
lang_code (PK)
lang_name
...
....

The Question is, if i want to add relationship between user and language table(Many to many) which is a right way ?

solution 1

user_language (Table Name)
--------------------------
user_id (FK)
lang_code (FK)


or solution 2

user_language (Table Name)
--------------------------
user_id (PK)
lang_code (PK)


or solution 3

user_language (Table Name)
--------------------------
user_lang_id (PK)
user_id (FK)
lang_code (FK)

i saw many people add primary key on many to many table, but i think that not important and wasting space. so which is right ?

btw i’m using PostgreSQL

Answer 1

Personally, I would go with solution 4:

user_language
---------------
user_id (FK)
lang_code (FK)

with composite PRIMARY KEY (user_id, lang_code)

I don’t think adding the surrogate key (user_lang_id in solution 3) really adds any value to the schema and simply adds another column to have to worry about. The primary key is a good idea though to maintain uniqueness – without it, you could add the same user_id/lang_code combination multiple times.