Question

We have a table (named QuestionBank) on which every question are stored :

Id     Topic     Hardness    Position
4        1          3           4
5        1          2           1
6        1          1           2
7        1          3           3
8        1          3           4
9        2          2           1
10       2          2           2
11       2          3           3
12       3          1           1
13       3          1           1
14       3          1           2

Every question belongs to a Topic and has it’s own Hardness and Position (every similar question based on their content have same position on corresponding Topic).

Note that the value in Position column is meaningless and it just partitions similar questions in a topic.

The goal is selecting N records with X records of Harness=1 and Y records of Hardness=2 and Z records of Hardness=3 and so on in a random manner; with this constraint that as much as possible there are no records with similar topic and position returned in result set.

For example, the records with Id=4 and Id=8 are similar in either Topic and Position hence one of them are desired to be in result set.

Answer 1

Here’s one way.

1. First if any Topic, Position have multiple questions assign each of these a random ordering RN1.
2. Then sort by that but randomly within each group. All the 1 values for RN1 will obviously be ordered first before getting onto 2 that denotes the second question for a particular Topic, Position combo.
3. Then use the result of that in a ROW_NUMBER calculation partitioned by Hardness to make the final SELECT easier.

.

WITH T1
     AS (SELECT *,
                Row_number() OVER (PARTITION BY Topic, Position 
                                       ORDER BY Newid()) AS RN1
         FROM   QuestionBank),
     T2
     AS (SELECT *,
                Row_number() OVER (ORDER BY RN1, Newid()) AS RN2
         FROM   T1),
     T3
     AS (SELECT *,
                Row_number() OVER (PARTITION BY Hardness 
                                       ORDER BY RN2) AS RN3
         FROM   T2)
SELECT Id,
       Topic,
       Hardness,
       Position
FROM   T3
WHERE  ( Hardness = 1
         AND RN3 <= 3 )
        OR ( Hardness = 2
             AND RN3 <= 2 )
        OR ( Hardness = 3
             AND RN3 <= 2 )
ORDER  BY Topic,
          Position,
          Hardness 

If you are only selecting a small proportion of the table each time there will likely be much more efficient ways however.