select random value based on probability chance

Question

How do I select a random row from the database based on the probability chance assigned to each row.
Example:

Make        Chance  Value
ALFA ROM         


        

Answer 1

You can do this by using rand() and then using a cumulative sum. Assuming they add up to 100%:

select t.*
from (select t.*, (@cumep := @cumep + chance) as cumep
      from t cross join
           (select @cumep := 0, @r := rand()) params
     ) t
where @r between cumep - chance and cumep
limit 1;

Notes:

• rand() is called once in a subquery to initialize a variable. Multiple calls to rand() are not desirable.
• There is a remote chance that the random number will be exactly on the boundary between two values. The limit 1 arbitrarily chooses 1.
• This could be made more efficient by stopping the subquery when cumep > @r.
• The values do not have to be in any particular order.
• This can be modified to handle chances where the sum is not equal to 1, but that would be another question.