Postgres most common value query - sql

Question

I am trying to figure out how to structure some queries, and I am a bit lost.
Tables:
CREATE TABLE dv_customer(
customer_id INTEGER PRIMARY KEY,
first_name VARCHAR(50),
last_name VARCHAR(50),
email VARCHAR(50),
address_id INTEGER,
active BOOLEAN
);
CREATE TABLE dv_address(
address_id INTEGER PRIMARY KEY,
address VARCHAR(50),
address2 VARCHAR(50),
district VARCHAR(50),
city_id INTEGER,
postal_code VARCHAR(50),
phone VARCHAR(50)
);
CREATE TYPE MPAA_RATING AS ENUM(
'G',
'PG',
'PG-13',
'R',
'NC-17'
);
CREATE TABLE dv_film(
film_id INTEGER PRIMARY KEY,
title VARCHAR(50),
description TEXT,
length SMALLINT,
rating MPAA_RATING,
release_year SMALLINT
);
CREATE TABLE cb_customers(
last_name VARCHAR(50),
first_name VARCHAR(50),
PRIMARY KEY (last_name, first_name)
);
CREATE TABLE cb_books(
title VARCHAR(50),
author_id INTEGER,
edition SMALLINT,
publisher VARCHAR(50),
PRIMARY KEY (title, author_id, edition)
);
CREATE TABLE cb_authors(
author_id INTEGER PRIMARY KEY,
first_name VARCHAR(50),
last_name VARCHAR(50)
);
CREATE TABLE mg_customers(
customer_id INTEGER PRIMARY KEY,
first_name VARCHAR(50),
last_name VARCHAR(50),
email VARCHAR(50),
address_id INTEGER,
active BOOLEAN
);
I need to figure out the following Queries:
What are the first and last names of all customers who live in the district having the most customers?
So far:
SELECT x.first_name, x.last_name
FROM dv_customer x, dv_address y
WHERE x.address_id = y.address_id
AND (SELECT count(district)
FROM dv_address >= SELECT count(district) FROM dv_address
);
What are the first and last names of the top 10 authors when ranked by the number of books each has written? I want author name and book count, in descending order of book count.
So far:
SELECT x.first_name, x.last_name, count(y.title)
FROM cb_authors x, cb_books y
GROUP BY first_name, last_name
ORDER BY count(*) DESC
LIMIT 10;
I know these are a bit of a mess, but they are the only queries I can't seem to figure out. Any help would be appreciated. I am a Postgres noob and just trying to figure out how it works.

Answer 1

What are the first and last names of the top 10 authors when ranked by the number of books each has written
This kind of query is typically done using a window function:
select first_name, last_name, num_books
from (
SELECT x.first_name, x.last_name,
dense_rank() over (order by count(y.title) desc) as rnk,
count(*) as num_books
FROM cb_authors x
join cb_books y on x.author_id = y.author_id
GROUP BY x.author_id
) t
where rnk <= 10
Your from clause FROM cb_authors x, cb_books y is missing a join condition and thus creates a cartesian join between the two tables. It is a good example on why the implicit joins in the where clause are a bad thing. If you get in the habit of using an explicit JOIN operator you will never accidentally miss a join condition.
The above also uses x.author_id which is sufficient for grouping as it is the primary key of the column and all other (non-grouped) columns in the select list are functionally dependent on it.

Answer 2

The query below will give you the district with the most customers
select district
from dv_address
group by district
order by count(*) desc
limit 1
Then you can select all customers living in that district with a sub query
select c.* from dv_customer c
join dv_address a on c.address_id = a.address_id
where a.district = (
select district
from dv_address
group by district
order by count(*) desc
limit 1
)
Similarly you can get the top 10 author_id's with this query
select author_id
from cb_books
group by author_id
order by count(*) desc
limit 10
Similarly, with a dervied table
select a.*, t.cnt from cb_authors a
join (
select author_id, count(*) cnt
from cb_books
group by author_id
order by count(*) desc
limit 10
) t on t.author_id = a.author_id
order by t.cnt desc