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 t

Answer 1

I have tried van's solution and, although it works, it is not quick.

My Solution

The way that I am solving this problem is by maintaining a separate, linked table for the weighting. The basic table structure is similar to this:

CREATE TABLE `table1` (
  `id` int(11) UNSIGNED AUTO_INCREMENT PRIMARY KEY,
  `name` varchar(100),
  `weight` tinyint(4) NOT NULL DEFAULT '1',
);

CREATE TABLE `table1_weight` (
  `id` bigint(20) UNSIGNED AUTO_INCREMENT PRIMARY KEY,
  `table1_id` int(11) NOT NULL
);

If I have a record in table1 with a weight of 3, then I create 3 records in table1_weight, linked to table1 via the table1_id field. Whatever the value of weight is in table1, that's how many linked records I create in table1_weight.

Testing

On a dataset with 976 records in table1 with a total weight of 2031 and hence 2031 records in table1_weight, I ran the following two SQLs:

1. A version of van's solution

   SELECT t.*
   FROM table1 t
   INNER JOIN
     ( SELECT t.id,
          SUM(tt.weight) AS cum_weight
      FROM table1 t
      INNER JOIN table1 tt ON tt.id <= t.id
      GROUP BY t.id) tc ON tc.id = t.id,
     ( SELECT SUM(weight) AS total_weight
      FROM table1) tt,
     ( SELECT RAND() AS rnd) r
   WHERE r.rnd * tt.total_weight <= tc.cum_weight
   ORDER BY t.id ASC
   LIMIT 1
   

2. Joining to a secondary table for the weighting

SELECT t.*
FROM table1 t
INNER JOIN table1_weight w
    ON w.table1_id = t.id
ORDER BY RAND()
LIMIT 1

SQL 1 takes consistently 0.4 seconds.

SQL 2 takes between 0.01 and 0.02 seconds.

Conclusion

If the speed of selection of a random, weighted record is not an issue, then the single table SQL suggested by van is fine and doesn't have the overhead of maintaining a separate table.

If, as in my case, a short selection time is critical, then I would recommend the two table method.