I wanted to calculate rank of each unique username where the final score is the average of all the correct answers in various attempts by a user. Also each category should be kept different.
I saw about creating Temporary tables but the main problem is how do i write a query which would find unique usernames calculate their average and rank them.
Below is a screenshot of the Database.
You could use avg and group by
select username, avg(correct)
from my_table
group by username
of if you need for each category
select username, category, avg(correct)
from my_table
group by username, category
Related
I have this table in mysql
I'm trying to select the usernames that has the most records in this database and display it on a website. So basically I want to see which username is logged the most and display that username along with the count. It's a voting system tracking the amount of people that vote. I would like to display the top five voters from this table. So the top 5 usernames that are repeated the most.
Therefore:
[4] davidxd33
will be displayed on the website because this user has four votes and is logged in the database four times.
I've tried SELECT username, count(username) FROM Votes which only returned the first name in the database, then the total count of usernames.
Try this:
SELECT username, COUNT(*)
FROM Votes
GROUP BY username
GROUP BY forces to return a line per username, and COUNT(*) counts the number of rows for each different username.
SELECT username,
count(*) as total
FROM Vote
group by username
Returns the top:
SELECT username, COUNT(*) AS total FROM Votes GROUP BY username ORDER BY total DESC
Thank you for the replies!
You should first group by usernames and then order them based on count in descending order and select the top 5. Hope this helps:-
select top 5 username, count(*) as votescount
from table t
group by username
order by votescount desc
I have a high scoring (top scores) system, which is calculating positions by players's eperience.
But now I need to use the player's rank in other places just the web, maybe more places in the web too like personal
high scores, and it will show the player's rank in that skill.
Therefore just looping & playing with the loop cycle like rank++ won't really work, cause I need to save that rank for
other places.
What I could do is loop through all players and then send a query to update that player's rank, but what if i have 1000 players? or more?
that means 1000 queries per load.
I have thought if there could be a SQL query I can use to do the same action, in one or two queries.
How can I do this? I calculate ranks by ordering by player's eperience, so my table structure looks like this:
Tables:
Players
id (auto_increment) integer(255)
displayname varchar(255) unique
rank integer(255) default null
experience bigint(255)
This should give you the rank for user with id = 1. If you want every player, just remove the WHERE clause:
SELECT a.id, a.displayname, a.rank, a.experience
FROM (
SELECT id, displayname, #r:=#r+1 AS rank, experience
FROM players, (SELECT #rank:=0) tmp
ORDER BY experience DESC) a
WHERE a.id = 1
I wouldn't have rank in the players table directly, since this would mean that you would have to recalculate it every time a user changes experience. You could do this query anytime you want to get the rank for a player or for a leaderboard.
If you still want to update it, You can do an INNER JOIN with this query to UPDATE the original table with the rank from this query.
in our project we've got an user table where userdata with name and different kind of scores (overall score, quest score etc. is stored). How the values are calculated doesn't matter, but take them as seperated.
Lets look table 'users' like below
id name score_overall score_trade score_quest
1 one 40000 10000 20000
2 two 20000 15000 0
3 three 30000 1000 50000
4 four 80000 60000 3000
For showing the scores there are then a dummy table and one table for each kind of score where the username is stored together with the point score and a rank. All the tables look the same but have different names.
id name score rank
They are seperated to allow the users to search and filter the tables. Lets say there is one row with the player "playerX" who has rank 60. So if I filter the score for "playerX" I only see this row, but with rank 60. That means the rank are "hard stored" and not only displayed dynamically via a rownumber or something like that.
The different score tables are filled via a cronjob (and under the use of a addional dummy table) which does the following:
copies the userdata to a dummy table
alters the dummy table by order by score
copies the dummy table to the specific score table so the AI primary key (rank) is automatically filled with the right values, representing the rank for each user.
That means: Wheren there are five specific scores there are also five score tables and the dummy table, making a total of 6.
How to optimize?
What I would like to do is to optimize the whole thing and to drop duplicate tables (and to avoid the dummy table if possible) to store all the score data in one table which has the following cols:
userid, overall_score, overall_rank, trade_score, trade_rank, quest_score, quest_rank
My question is now how I could do this the best way and is there another way as the one shown above (with all the different tables)? MYSQL-Statements and/or php-code is welcome.
Some time ago I tried using row numbers but this doesn't work a) because they can't be used in insert statements and b) because when filtering every player (like 'playerX' in the example above) would be on rank 1 as it's the only row returning.
Well, you can try creating a table with the following configuration:
id | name | score_overall | score_trade | score_quest | overall_rank | trade_rank | quest_rank
If you do that, you can use the following query to populate the table:
SET #overall_rank:=-(SELECT COUNT(id) FROM users);
SET #trade_rank:=#overall_rank;
SET #quest_rank:=#overall_rank;
SELECT *
FROM users u
INNER JOIN (SELECT id,
#overall_rank:=#overall_rank+1 AS overall_rank
FROM users
ORDER BY score_overall DESC) ovr
ON u.id = ovr.id
INNER JOIN (SELECT id,
#trade_rank:=#trade_rank+1 AS trade_rank
FROM users
ORDER BY score_trade DESC) tr
ON u.id = tr.id
INNER JOIN (SELECT id,
#quest_rank:=#quest_rank+1 AS quest_rank
FROM users
ORDER BY score_quest DESC) qr
ON u.id = qr.id
ORDER BY u.id ASC
I've prepared an SQL-fiddle for you.
Although I think performance will weigh in if you start getting a lot of records.
A bit of explanation: the #*_rank things are SQL variables. They get increased with 1 on every new row.
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.
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.
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.
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
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.
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
Now I have a question on mysql paging.
A user's record is displayed on a table with many other user's record. and the table is sorted/paged.
Now i need to display the page that containing the user's row directly after the user login. How can I achieve this?
A simple thought would be firstly find out the rownum of the user, then do the paging accordingly, but I'm wondering if there are better ways to do it.
thanks.
Sample table for discussion
create table t_users (id int auto_increment primary key, username varchar(100));
insert t_users(username) values
('jim'),('bob'),('john'),('tim'),('tom'),
('mary'),('elise'),('karl'),('karla'),('bob'),
('jack'),('jacky'),('jon'),('tobias'),('peter');
The query to show the page on which the user is on, not specifically putting the user to the top of the page (which would have been a lot easier)
select username, id
from
(select count(*) pos
from t_users
where username <= 'tobias') pos,
(select #row:=#row+1 row, u.*
from (select #row:=0) initvars, t_users u
order by u.username, u.id) numbered
where floor((numbered.row + 3)/4) = floor((pos.pos+3)/4);
Notes:
The page size here is 4, the number 3 is the result of taking 1 off the page size
The username of the user who just logged in is 'tobias', which is used in the first sub-query
There had better be an index along the ORDER BY clause (in this case on username)
This will cause a table scan to fully row-number the last subquery
select count(*)
FROM table
WHERE [sort_by_field] < [actual_records_value]