SQL random sample with groups

Question

I have a university graduate database and would like to extract a random sample of data of around 1000 records.

I want to ensure the sample is representative of the population so would like to include the same proportions of courses eg

I could do this using the following:

select top 500 id from degree where coursecode = 1 order by newid()
union
select top 300 id from degree where coursecode = 2 order by newid()
union
select top 200 id from degree where coursecode = 3 order by newid()

but we have hundreds of courses codes so this would be time consuming and I would like to be able to reuse this code for different sample sizes and don't particularly want to go through the query and hard code the sample sizes.

Any help would be greatly appreciated

Answer 1

Add a table for storing population.

I think it should be like this:

SELECT *
FROM (
    SELECT id, coursecode, ROW_NUMBER() OVER (PARTITION BY coursecode ORDER BY NEWID()) AS rn
    FROM degree) t
    LEFT OUTER JOIN
    population p ON t.coursecode = p.coursecode
WHERE
    rn <= p.SampleSize

Answer 2

You want a stratified sample. I would recommend doing this by sorting the data by course code and doing an nth sample. Here is one method that works best if you have a large population size:

select d.*
from (select d.*,
             row_number() over (order by coursecode, newid) as seqnum,
             count(*) over () as cnt
      from degree d
     ) d
where seqnum % (cnt / 500) = 1;

EDIT:

You can also calculate the population size for each group "on the fly":

select d.*
from (select d.*,
             row_number() over (partition by coursecode order by newid) as seqnum,
             count(*) over () as cnt,
             count(*) over (partition by coursecode) as cc_cnt
      from degree d
     ) d
where seqnum < 500 * (cc_cnt * 1.0 / cnt)