select RAND() with probability - php

Question

I want to display 10 lines of the "questions" table with probability 0.2 of question that have type_id = 1 and probability 0.8 of question that have type_id =2.
Below my request, how to add the probability?
$query = "select * from questions ORDER BY RAND() LIMIT 10";
I want to display 10 questions which 20% of the questions have type_id = 2 and 80% have type_id = 1.
can someone help me please?

Answer 1

As I noted in the comments, you won't be able to use anything as obvious as ORDER BY RAND() if you want to include probabilities or anything like that. ORDER BY RAND() simply doesn't support that kind of thing. ORDER BY RAND() is also very slow, and not really suitable for use on a database of any significant size anyway.
There are a whole bunch of approaches you can use to do a random sort order with weighting or probabilities; I'm not going to try to discuss them all; I'll just give you a relatively simple one, but please be aware that the best technique for you will depend on your specific use case.
A simple approach would be something like this:
Create a new integer field on your table called weight or something similar.
Add a DB index for this field to enable you to query it quickly.
Set the first record to a value equal to its weighting as a whole number. ie a probability of 0.2 could be a weight of 20.
Set each subsequent record to the max value of this field plus the weight for that record. So if the second record is also 0.2, it would get a value of 40; if the one after that is only 0.1, it would be 50; and so on.
Do likewise for any new records that get added.
Now you can select a random record, with different weights for each record, as follows:
SELECT * FROM questions
WHERE weight >= FLOOR(RAND() * (SELECT MAX(weight) FROM questions))
ORDER BY weight
LIMIT 1
(note, I'm writing is answer in a hurry and without resource to test it; I haven't run this query so I may have got the syntax wrong, but the basic technique is sound)
This will pick a random number between zero and the largest weight value, and then find the question record that has the closest weight value to that random number.
Also, because the weight field is indexed, this query will be quick and efficient.
Downsides of this technique: It assumes that the weights for any given record won't change. If the weight of a record does need to change, then you would have to update the weight value for every record after it in the index.
[EDIT]
Let's imagine a table like this:
id Name
1 Question One
2 Question Two
3 Question Three
4 Question Four
5 Question Five
In this example, we want Questions 1 and 2 to have a probability of 0.2, question 3 to have a probability of 0.1 and questions 4 and 5 to have a probability of 0.3. Those probabilities can be expressed as integers by multiplying them by 100. (multiply by 10 also works, but 100 means we can have probabilities like 0.15 as well)
We add the weight column and the index for it, and set the weight values as follows:
id Name Weight
1 Question One 20
2 Question Two 40 (ie previous value + 20)
3 Question Three 50 (ie previous value + 10)
4 Question Four 80 (ie previous value + 30)
5 Question Five 110 (ie previous value + 30)
Now we can run our query.
The random part of the query FLOOR(RAND() * (SELECT MAX(weight) FROM questions)) will select a value between zero and 110. Let's imagine it gives 68.
Now the rest of our query says to pick the first record where the weight is greater than 68. In this case, that means that the record we get is record #4.
This gives us our probability because the random number could be anything, but is more likely to select a given record if the gap between its weight and the one before it is larger. You'll get record #4 three times as often as record #3.