Users Rating update, recalculation - php

Question

I have a table with fields id, votes(for each users), rating.
Task: Counting user rating based on votes for him and for others. that is, each time i update the field votes needed recalculation field rating.
Which means some can be on the 3rd place. voted for him and that he would be stood up to 2rd place, and the other vice versa - from 2 to 3. (in rating fiels)
How to solve this problem? Each time update the field to count users ratings on php and do a lot of update query in mysql is very expensive.

Answer 1

If you want to get the ratings with a select without having a rating column, then this is the way. However from a performance perspective I cannot guarantee this will be your best option. The way it works is that if two users have the same amount of votes they will have the same rating and then it will skip ahead the necessary number for the next different rating:
set #rating:=0;
set #count:=1;
select id,
case when #votes<>votes then #rating:=#rating+#count
else #rating end as rating,
case when #votes=votes then #count:=#count+1
else #count:=1 end as count,
#votes:=votes as votes
from t1
order by votes desc
sqlfiddle
This gives you an extra column which you can ignore, or you could wrap this select in to a subquery and have:
select t2.id,t2.votes,t2.rating from (
select id,
case when #votes<>votes then #rating:=#rating+#count
else #rating end as rating,
case when #votes=votes then #count:=#count+1
else #count:=1 end as count,
#votes:=votes as votes
from t1
order by votes desc) as t2
but the sqlfiddle is strangely giving inconsistent results so you'd have to do some testing. If anyone knows why this is I'd be interested in knowing the reason.
If you want to get the rating for just one user then doing the subquery option and using a where after the from should give you the desired result. sqlfiddle - but again, inconsistent results, run it a few times and sometimes it gives rating as 10 other times as 30. I think testing in your db to see what happens will be best.

Answer 2

Well it depends on a lot of factors
Do you have a large system that is growing exponentially?
Do you require the voting data for historical reporting?
Do users need to register when they vote?
Will this system be use only for one voting type throughout the system life cycle or will more voting on different subjects take place?
If all of the answers are NO then your current update method will work just fine. Just ensure that you apply best coding and MySQL table practices anyway.
Let assume most or all your answers were YES then I would suggest the following:
Every time a vote takes place INSERT the record into your table
Using INSERT, add a timestamp, user id if not possible then maybe an ip address/location
Assign a subject id as foreign key from the vote_subject table. In this table store the subject and date of voting
Now you can create a SELECT statement that can count the votes and calculate the ratings. The person top of the vote count list will get rating 1 in the SELECT. Furthermore you can filter per subject, per day, per user and you should also be able to determine volume depending on the result required.
All this of course dependent on how your system will scale in future. This might be way overkill but something to think about.

Answer 3

Yes aggregations are expensive. You could update a rank table every five minutes or so and query from there. The query as you probably already now is this:
select id, count(*) as votes
from users
group by id
order by votes desc

Answer 4

Instead of having the fields id, votes and rating, alter the table to have the fields id, rating_sum and rating_count. Each time you have a new rating you quering the database like this:
"UPDATE `ratings` SET `rating_count` = `rating_count` + 1, `rating_sum` = `rating_sum`+ $user_rating WHERE `id` = $id"
Now the rating is just the average -> rating_sum / rating_count. No need to have a field with the rating.
Also, to prevent a user rate more than one times, you could create a table named rating_users that will have 2 foreign keys the users.id and ratings.id. The primary key will be (users.id, ratings.id). So each time a user tries to rate first you check this table.

Answer 5

I would recommend doing this when querying the data. It would be much simpler. Order by votes descending.
Perhaps create a view and use the view when querying the data.
You could try something like this:
SET #rank := 0
select id, count(*) as votes, #rank := #rank + 1
from users
group by id
order by votes desc
Or
SET #rank := 0
select id, votes, #rank := #rank + 1
from users
order by votes desc