How to select one row randomly taking into account a weight?

Question

I have a table which looks like that:

id: primary key
content: varchar
weight: int

What I want to do is randomly select one row from this table, but taking into account the weight. For example, if I have 3 rows:

id, content, weight
1, "some content", 60
2, "other content", 40
3, "something", 100

The first row has 30% chance of being selected, the second row has 20% chance of being selected, and the third row has 50% chance of being selected.

Is there a way to do that? If I have to execute 2 or 3 queries it's not a problem.

Answer 1

I think the simplest is actually to use the weighted reservoir sampling:

SELECT
  id,
  -LOG(RAND()) / weight AS priority
FROM
  your_table
ORDER BY priority
LIMIT 1;

It's a great method that lets you choose M out of N elements where the probability to be chosen for each element is proportional to its weight. It works just as well when you happen to only want one element. The method is described in this article. Note that they choose the biggest values of POW(RAND(), 1/weight), which is equivalent to choosing the smallest values of -LOG(RAND()) / weight.