Random values with different weights - asp.net-mvc

Question

Here's a question about entity framework that has been bugging me for a bit.
I have a table called prizes that has different prizes. Some with higher and some with lower monetary values. A simple representation of it would be as such:
+----+---------+--------+
| id | name | weight |
+----+---------+--------+
| 1 | Prize 1 | 80 |
| 2 | Prize 2 | 15 |
| 3 | Prize 3 | 5 |
+----+---------+--------+
Weight is this case is the likely hood I would like this item to be randomly selected.
I select one random prize at a time like so:
var prize = db.Prizes.OrderBy(r => Guid.NewGuid()).Take(1).First();
What I would like to do is use the weight to determine the likelihood of a random item being returned, so Prize 1 would return 80% of the time, Prize 2 15% and so on.
I thought that one way of doing that would be by having the prize on the database as many times as the weight. That way having 80 times Prize 1 would have a higher likelihood of being returned when compared to Prize 3, but this is not necessarily exact.
There has to be a better way of doing this, so i was wondering if you could help me out with this.
Thanks in advance

Answer 1

Normally I would not do this in database, but rather use code to solve the problem.
In your case, I would generate a random number within 1 to 100. If the number generated is between 1 to 80 then 1st one wins, if it's between 81 to 95 then 2nd one wins, and if between 96 to 100 the last one win.
Because the random number could be any number from 1 to 100, each number has 1% of chance to be hit, then you can manage the winning chance by giving the range of what the random number falls into.
Hope this helps.
Henry

Answer 2

This can be done by creating bins for the three (generally, n) items and then choose a selected random to be dropped in one of those bins.
There might be a statistical library that could do this for you i.e. proportionately select a bin from n bins.
A solution that does not limit you to three prizes/weights could be implemented like below:
//All Prizes in the Database
var allRows = db.Prizes.ToList();
//All Weight Values
var weights = db.Prizes.Select(p => new { p.Weight });
//Sum of Weights
var weightsSum = weights.AsEnumerable().Sum(w => w.Weight);
//Construct Bins e.g. [0, 80, 95, 100]
//Three Bins are: (0-80],(80-95],(95-100]
int[] bins = new int[weights.Count() + 1];
int start = 0;
bins[start] = 0;
foreach (var weight in weights) {
start++;
bins[start] = bins[start - 1] + weight.Weight;
}
//Generate a random number between 1 and weightsSum (inclusive)
Random rnd = new Random();
int selection = rnd.Next(1, weightsSum + 1);
//Assign random number to the bin
int chosenBin = 0;
for (chosenBin = 0; chosenBin < bins.Length; chosenBin++)
{
if (bins[chosenBin] < selection && selection <= bins[chosenBin + 1])
{
break;
}
}
//Announce the Prize
Console.WriteLine("You have won: " + allRows.ElementAt(chosenBin));