Create a random selection weighted on number of points, SQL - sql

Question

I have a table of winners for a prize draw, where each winner has earned a number of points over the year. There are 1300 registered users, with points varying between 50 and 43,000. I need to be able to select a random winner, which is straight forward, but the challenge I am having is building the logic where each point counts as an entry ticket into the prize draw. Would appreciate any help.
John

Answer 1

Your script would look something similar to this:
Script 1 :
DECLARE #Name varchar(100),
#Points int,
#i int
DECLARE Cursor1 CURSOR FOR SELECT Name, Points FROM Table1
OPEN Cursor1
FETCH NEXT FROM Cursor1
INTO #Name, #Points
WHILE ##FETCH_STATUS = 0
BEGIN
SET #i = 0
WHILE #i < #Points
BEGIN
INSERT INTO Table2 (Name)
VALUES (#Name)
SET #i = #i + 1
END
FETCH NEXT FROM Cursor1 INTO #Name, #Points
END
DEALLOCATE Cursor1
I have created a table (Table1) with only a Name and Points column (varchar(100) and int), I have created a cursor in order to look through all the records within Table1 and then loop through the Points and then inserted each record into another table (Table2).
This then imports the Name depending on the Points column.
Script 2 :
DECLARE #Name varchar(100),
#Points int,
#i int,
#Count int
CREATE TABLE #temptable(
UserEmailID nvarchar(200),
Points int)
DECLARE Cursor1 CURSOR FOR SELECT UserEmailID, Points FROM Table1_TEST
OPEN Cursor1
FETCH NEXT FROM Cursor1
INTO #Name, #Points
WHILE ##FETCH_STATUS = 0
BEGIN
SET #i = 0
WHILE #i < #Points
BEGIN
INSERT INTO #temptable (UserEmailID, Points)
VALUES (#Name, #Points)
SET #i = #i + 1
END
FETCH NEXT FROM Cursor1 INTO #Name, #Points
END
DEALLOCATE Cursor1
SELECT * FROM #temptable
DROP TABLE #temptable
In Script2 I have imported the result into a TEMP table as requested.
The script now runs through each record within you Table1 and imports the individuals UserEmailID and Points into the TEMP table depending on how much the Points are in Table1.
So if John has a total of 3 points, and Sarah 2, the script will import Johns UserEmailID 3 times into the TEMP table and 2 times for Sarah.
If you apply the random selector on the TEMP table, it will then randomly select a individual.
John would obviously stand a better chance to win because he has 3 records in the TEMP table whereas Sarah only has 2.
Suppose Johns UserEmailID is 1 and Sarah is 2:
The OUTPUT of TEMP table would then be:
UserEmailID | Points
1 | 3
1 | 3
1 | 3
2 | 2
2 | 2
Please let me know if you need any clarity.
Hope this helps.

Answer 2

You can do a weighted draw using the following method:
Calculate the cumulative sum of points.
Divide by the total number of points to get a value between 0 and 1
Each row in the original data will have a range, such as [0, 0.1), [0.1, 0.3), [0.3, 1]
Calculate a random number and choose the row where the value falls in the range
Here is standard'ish SQL for this approach:
with u as (
select u.*,
coalesce(lead(rangestart) over (order by points) as rangeend, 1)
from (select u.*,
sum(points*1.0) over (order by points) / sum(points) over () as rangestart
from users u
) u
),
r as (
select random() as rand
)
select u.*
from u
where r.rand between rangestart and rangeend;
In addition to using window functions (which can be handled by correlated subqueries in many cases), the exact format depends on whether the random number generator is deterministic for a query (such as SQL Server where random() returns one value no matter how often called in a query) or non-deterministic (such as in other databases). This method only requires one value for the random number generator, so it will work with either method.

Answer 3

So you want a winner with 1000 points have double the chances as another with only 500 points.
Sort the winners by whatever order and create a running total for the points:
id points
winner1 100
winner2 50
winner3 150
gives:
id points from to
winner1 100 1 100
winner2 50 101 150
winner3 150 151 300
Then compare with a random number from 1 to sum(points), in the example a number between 1 and 300. Find the winner with that number range and you're done.
select winpoints.id_winner
from
(
select
id as id_winner,
coalesce(sum(points) over(order by id rows between unbounded preceding and 1 preceding), 0) + 1 as from_points,
sum(points) over(order by id rows between unbounded preceding and current row) as to_points
from winners
) winpoints
where (select floor(rand() * (sum(points) from winners)) + 1)
between winpoints.from_points and winpoints.to_points;

Answer 4

This solution also works with fractional points/weights. It creates a helper table usersum.
create table user (id int primary key, points float);
insert into user values (1, 0.5), (2, 0), (3, 1);
create table usersum (id int primary key, pointsum float);
insert into usersum
select id, (select sum(points) from user b where b.id <= a.id)
from user a;
set #r = rand() * (select max(pointsum) from usersum);
select #r, usersum.* from usersum where pointsum >= #r order by id limit 1;
http://sqlfiddle.com/#!2/ae539e/1