What is the most efficient way in T-SQL to compare answer strings to answer keys for scoring an exam

Question

These exams typically have about 120 questions. Currently, they strings are compared to the keys and a value of 1 or 0 assigned. When complete, total the 1's for a raw score.

Are there any T-SQL functions like intersect or diff or something all together different that would handle this process as quickly as possible for 100,000 examinees?

Thanks in advance for your expertise.

-Steven

Answer 1

Try selecting the equality of a question to its correct answer. I assume you have the student's tests in one table and the key in another; something like this ought to work:

select student_test.student_id, 
   student_test.test_id, 
   student_test.question_id, 
   (student_test.answer == test_key.answer OR (student_test.answer IS NULL AND test_key.answer IS NULL))
from student_test
INNER JOIN test_key
   ON student_test.test_id = test_key.test_id
      AND student_test.question_id = test_key.question_id
WHERE student_test.test_id = <the test to grade>

You can group the results by student and test, then sum the last column if you want the DB to give you the total score. This will give a detailed "right/wrong" analysis of the test.

EDIT: The answers being stored as a continuous string make it much harder. You will most likely have to implement this in a procedural fashion with a cursor, meaning each student's answers are loaded, SUBSTRINGed into varchar(1)s, and compared to the key in an RBAR (row by agonizing row) fashion. You could also implement a scalar-valued function that compared string A to string B one character at a time and returned the number of differences, then call that function from a driving query that will call this function for each student.

Answer 2

Something like this might work out for you:

select student_id, studentname, answers, 0 as score
    into #scores from test_answers
declare @studentid int
declare @i int
declare @answers varchar(120)
declare @testkey varchar(120)
select @testkey = test_key from test_keys where test_id = 1234
declare student_cursor cursor for
    select student_id from #scores
open student_cursor
fetch next from student_cursor into @studentid
while @@FETCH_STATUS = 0
begin
    select @i = 1
    select @answers = answers from #scores where student_id = @studentid
    while @i < len(@answers)
    begin
        if mid(@answers, @i, 1) = mid(@testkey, @i, 1)
            update #scores set score = score + 1 where student_id = @studentid
        select @i = @i + 1
    end
    fetch next from student_cursor into @studentid
end
select * from #scores
drop table #scores

I doubt that's the single most efficient way to do it, but it's not a bad starting point at least.