SQL select 语句带随机数

Question

I'm currently creating some type of quiz website.

My plan: Creating a random number in my sql statement to get a random question from my table.

Eg every question have a numeric id -> creating random number (max number = records in my table) -> select random question by id

I tried the followed statement:

SELECT *
FROM question
WHERE ID = (SELECT FLOOR(RAND() * (SELECT COUNT(ID) FROM question)) + 1);

My problem: Sometimes I got no result, sometimes I got two results and sometimes it worked as planed.

If I try the SELECT FLOOR etc. on it's own, the random number works perfectly.

Any suggestions? Thank you in advance my friends!

Answer 1

If you don't have very many questions, you can simply do:

select q.*
from question q
order by rand()
limit 1;

If you have lots of questions, then reducing the number is important for performance. Something like:

select q.*
from question q cross join
     (select count(*) from q) qq  
where rand() < 100 / q   -- get a sample of about 100
order by rand()
limit 1;