multiple foreign keys as primary key postgres, should I do it? - sql

Question

This is one of those: why I should or why I should not.
So my books app has reviews, but one user must not review the same book more than one time. In my point of view makes sense to create a table for the reviews and make the user_id and the book_id (ISBN) as the PRIMARY KEY for the reviews table. But, could it be a problem at some point if the application gets too many reviews, for example, could that decision slow the queries?
I am using postgres and I am not sure if the following code is correct:
CREATE TABLE users(
user_id PRIMARY KEY SERIAL,
user_name VARCHAR NOT NULL UNIQUE,
pass_hash VARCHAR NOT NULL,
email VARCHAR NOT NULL UNIQUE,
);
CREATE TABLE books(
book_id PRIMARY KEY BIGINT,
author VARCHAR NOT NULL,
title VARCHAR NOT NULL,
year INT NOT NULL CHECK (year > 1484),
review_count INT DEFAULT 0 NOT NULL,
avrg_score FLOAT,
);
CREATE TABLE reviews(
user_id INT FOREIGN KEY REFERENCES users(user_id) NOT NULL
book_id INT FOREIGN KEY REFERENCES books(book_id) NOT NULL
score INT NOT NULL CHECK (score > 0, score < 11)
PRIMARY KEY (book_id, user_id)
);

Answer 1

This is a perfectly valid design choice.
You have a many-to-many relationship between books and users, which is represented by the reviews table. Having a compound primary key based on two foreign keys lets you enforce referential integrity (a given tuple may only appear once), and at the same time provide a primary key for the table.
Another option would be to have a surrogate primary key for the bridge table. This could make things easier if you need to reference the reviews from another table, but you would still need a unique constraint on both foreign key columns for integrity, so this would actually result in extra space being used.
When it comes to your code, it has a few issues:
the primary key keyword goes after the datatype
the check constraint is incorrectly formed
missing or additional commas here and there
Consider:
CREATE TABLE users(
user_id SERIAL PRIMARY KEY ,
user_name VARCHAR NOT NULL UNIQUE,
pass_hash VARCHAR NOT NULL,
email VARCHAR NOT NULL UNIQUE
);
CREATE TABLE books(
book_id BIGINT PRIMARY KEY,
author VARCHAR NOT NULL,
title VARCHAR NOT NULL,
year INT NOT NULL CHECK (year > 1484),
review_count INT DEFAULT 0 NOT NULL,
avrg_score FLOAT
);
CREATE TABLE reviews(
user_id INT REFERENCES users(user_id) NOT NULL,
book_id INT REFERENCES books(book_id) NOT NULL,
score INT NOT NULL CHECK (score > 0 and score < 11),
PRIMARY KEY (book_id, user_id)
);

Answer 2

I the above is good but I would drop columns review_count and avrg_score from books. These are derivable when needed. If needed for your application then instead of storing these create a view to derive them. This avoids the always complicated process of maintaining running values:
create view books_vw as
select b.book_id
, b.author
, b.title
, b.year
, count(r.*) review_count
, avg(r.score) avrg_score
from books b
left join reviews r
on r.book_id = b.book_id
group by
b.book_id
, b.author
, b.title
, b.year
;