CODEWITHSUNDEEP
phpmysqlsqlalgorithm
user1005424
user1005424

Reputation: 67

Selecting MySQL records with chance

I have a MySQL table with 3 fields as follows.

id, name, chance
1001, name1, 1
1002, name2, 3
1003, name3, 1

I want to randomly select a record 100 times. Out of 100 times, I want record id 1001 to be selected 20 times (1/5 chance), record id 1002 to be selected 60 times (3/5 chance), and record id 1003 to be selected 20 times (1/5 chance).

How to do that in MySQL and/or PHP?

Many thanks in advance!

Upvotes: 1

Views: 277

Answers (2)

Gordon Linoff
Gordon Linoff

Reputation: 1270401

Doing this in SQL is a bit challenging. If your numbers are really so small, the easiest way is by doing a cross join to multiply out the records and then take a random row from there:

select t.*
from t join
     (select 1 as n union all select 2 union all select 3) n
     on n.n <= t.chance
order by rand()
limit 1;

This works if "chance" is a small integer.

Otherwise, I think you need a cumulative sum of chance and then a comparison. Something like:

select t.*
from (select t.*, (@sumchance := @sumchance + chance) as sumchance
      from t cross join (select @sumchance := 0) const
     ) t cross join
     (select sum(chance) as totchance from t) tot
where rand()*totchance between sumchance - chance and sumchance
limit 1;

This calculates the sum of chance up to a given row (the order doesn't make a difference because this is random). It then calculates a random number in the same range and compares it to sumchance - chance and chance. This should return one row, but there is a boundary case where rand()*totchance exactly equals sumchance. Either row could be returned. The limit 1 limits it to one row.

Upvotes: 2

JD3
JD3

Reputation: 410

To generate a random number in php use

int rand ( int $min , int $max )

Then use a series of if statements.

For example:

$random = rand (1,100);
(INSERT LOOP)
if ($random <= 20){
$id1001 = $id1001 + 1;
}
else if ($random > 20 and $random < 80){
$id1002 = $id1002 + 1;
}
else if ($random > 80 and $random < 100){
$id1003 = $id1003 + 1;
}
(END LOOP)

Upvotes: 2

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