SQL Query to find the value which has max number of occurrences in a table without nesting

Question

I'm working on the following schema (bold text stands for pk, and ":" stand for referenced tables) :

USERS(username, name, surname);

PRODUCTS(id, name, quantity);

PURCHASES(user:USERS, product:PRODUCTS, dateAndTime, quantityPurchased);

I want to find name and surname of the user who has made the max number of purchases.

Firstly I use a nested query to find out the number of purchases for each user and then I select the user which purchased >= ALL those values:

SELECT name, surname, username
FROM users JOIN purchases ON username = user
GROUP BY name, surname, username
HAVING count(*) >= ALL(
    SELECT count(*)
    FROM utenti JOIN acquisti ON username = user
    GROUP BY username)

Is there another way to achieve the same without using nested queries?

Thank you in advance for your time.

Answer 1

Yes there is. This sounds like a homework assignment, but you seem to have put some work into it. The idea is to order by the count(*) and take the first row. The syntax in SQL Server, Sybase, and Access is:

SELECT top 1 name, surname, username
FROM users as u INNER JOIN
     purchases as p
     ON u.username = p.user
GROUP BY name, surname, username
ORDER BY count(*) desc;

Other databases would put a limit 1 clause after the order by, instead of top 1. And others may have even more arcane syntax.

Answer 2

This will return the user with most purchased items.

SELECT TOP 1
   U.name, U.surname, SUM(P.quantityPurchased) Purchases
FROM users U
  JOIN purchases P
    on U.username= P.user
GROUP BY U.username, U.name, U.surname
ORDER BY Purchases DESC

and this will return the user with most purchaes.

SELECT TOP 1
   U.name, U.surname, COUNT(*) occurrences
FROM users U
  JOIN purchases P
    on U.username= P.user
GROUP BY U.username, U.name, U.surname
ORDER BY occurrences DESC