select a random rows from table - mysql

Question

I have two tables one called topic as shown below!
topic_id topic_name
1 topic 1
2 topic 2
3 topic 3
and another table called questions as shown
q_id question_name topic_id
1 question 1 1
2 question 2 1
3 question 3 1
4 question 4 2
5 question 5 2
6 question 6 2
7 question 7 3
8 question 8 3
9 question 9 3
i want to choose random 2 question from given three topic. Someone please help me to fix this issue

Answer 1

Get list of topics with their question IDs GROUP_CONCAT([column] order by RAND()).
And then link table to itself.
SELECT t.q_id, t.question_name, t.topic_id
FROM table t
JOIN (
SELECT topic_id, SUBSTRING_INDEX(GROUP_CONCAT(q_id ORDER BY RAND()), ',', 2) as qList
FROM table GROUP BY topic_id
) tGrouped ON FIND_IN_SET(t.q_id, tGrouped.qList)>0

Answer 2

One can sort the rows randomly and then fetch the top row from this random order
For two random questions which could have same topic:
SELECT * FROM questions
ORDER BY RAND()
LIMIT 2
For two random questions which should have different topic:
Use 2 different queries which take as parameters two different topic_ids (t1, t2):
First select 2 random topic ids (similarly to above code):
SELECT topic_id FROM topics
ORDER BY RAND()
LIMIT 2
Then select 2 random questions with these topics ids (2 select statements)
SELECT * FROM questions
WHERE topic_id = t1
ORDER BY RAND()
LIMIT 1
SELECT * FROM questions
WHERE topic_id = t2
ORDER BY RAND()
LIMIT 1
UPDATE (after OP's comment and explanation)
To obtain two random questions from every topic use a variation of the above solutions:
3 select statements (one for each topic):
SELECT * FROM questions
WHERE topic_id = needed_topic_id_here
ORDER BY RAND()
LIMIT 2
repeat the select for every topic_id.
Presumably these select statements could be combined into one big select statement, but i'm not sure at this point.
Note as pointed out in another answer, this could be less efficient (to randomly select in pure sql) and a better solution would be to pre-compute random indices in PHP (or whatever your platform is) and then actually select the random questions. Since no language is mentioned in the question, i'll leave it here (and point to the other answer(s) for this approach)

Answer 3

You can use ORDER BY RAND() and LIMIT 2 in the query but it runs painfully slow for tables that have thousand of records or more.
A better approach for big tables is to get the bounding values of the PK field using the WHERE condition you need, generate 2 random numbers smaller between these bounding values in PHP then issue 2 MySQL queries to get 2 questions.
Something along these lines:
$query = '
SELECT MIN(q_id) AS min_id, MAX(q_id) AS max_id
FROM questions
WHERE topic_id = 1 # put the filtering you need here
';
// Run the query
// ... use your regular PHP code for database access here ...
// get and store the returned values in PHP variables $minId and $maxId
// Keep the generated random values here to avoid duplicates
$list = array();
// Get N random questions from the database
for ($cnt = 0; $cnt < N; $cnt ++) {
// Generate a new ID that is not in the list
do {
$id = rand($minId, $maxId);
} while (in_array($id, $list);
// Put it into the list to avoid generating it again
$list[] = $id;
// Get the random question
$query = "
SELECT *
FROM questions
WHERE topic_id = 1
AND q_id <= $id
ORDER BY q_id DESC
LIMIT 1
";
// Run the query, get the question
// ... use your regular PHP code for database access here ...
}
No matter what queries you run (these or others provided by other answer), you need indexes on q_id and the columns used in the WHERE clause.
I hope q_id is the PK of the table which means it already is an UNIQUE INDEX.

Answer 4

Provide 3 topic ids to get 2 questions at random:
select
q.question_name
from
topics t,
questions q
where
t.topic_id = q.topic_id and
t.topic_id in (1, 2, 3) /*define your 3 given topics*/
order by rand() limit 0,2;