SQL如何连接这些表

Question

The scenario:

I have a website which let users vote between cars which they like most. Cars are saved in the table cars , votes are saved in votes and the column country_id from the table cars reference to countries (where the carbrand comes from).

I want to show the users which country has the most votes. Simple version of the tables:

CARS

id
name
country_id

Countries

id
name

Votes

id
user_id
car_id

Ideally I would like to show the users the top x countries. And how many votes they all have.

Bonus: would it be possible to use this query for a certain user? So they see their top x with countries they voted on.

And which indexes you suggest? The votes table can grow beyond 10 million votes, the cars table can grow fast too.

Answer 1

I think you can achieve this with a LEFT JOIN query and GROUP BY aggregate function

SELECT COUNT(a.id) as total_votes, c.name as country_name
FROM Votes a
LEFT JOIN CARS b
ON a.car_id = b.id
LEFT JOIN Countries c
ON b.country_id = c.id
GROUP BY b.name, c.name
ORDER BY total_votes DESC

Answer 2

Indexes on Cars.CountryID, Votes.UserID and Votes.CarID would seem reasonable. As mzedler suggested though, when you get up to tens of millions, aggregates can be a bad idea.

There are number of ways of addressing that, triggers, a cache, or adding date voted to votes, so you break down the number of records you have to count in one go. eg cache votes daily and then just query those made since midnight and then sum them.