Combining two queries into one query

Question

Query 1

select test_id, count (student_id)*100/ 
  (select count(distinct student_id)from test_taken)as "pass rate"
from test_taken
where result>50
group by test_id;

Query 2

 select test_id, count (student_id)*100/ 
    (select count(distinct student_id)from test_taken)as "fail rate"
 from test_taken
 where result<50
 group by test_id;

I have the following Table:

test_taken
Columns:test_id,student_id,result

I am looking to get the percentage pass rate and fail rate where a pass is result >50% and fail is result <50%.

I have the pass rate and fail rate as 2 separate queries but I want them combined into one query.

Answer 1

SELECT test_id, 
       sum(case when result > 50 then 1 else 0 end) * 100 / (SELECT COUNT(DISTINCT student_id) 
                                   FROM   test_taken) AS "pass rate",
       sum(case when result < 50 then 1 else 0 end) * 100 / (SELECT COUNT(DISTINCT student_id) 
                                   FROM   test_taken) AS "fail rate" 
FROM   test_taken 
GROUP  BY test_id; 

Answer 2

If the result of the two queries is equal in terms of column numbers and column types, you can use UNION to get one table result :

SELECT test_id, 
       COUNT (student_id) * 100 / (SELECT COUNT(DISTINCT student_id) 
                               FROM   test_taken)AS rate 
FROM   test_taken 
WHERE  result > 50 
GROUP  BY test_id; 

UNION

SELECT test_id, 
       COUNT (student_id) * 100 / (SELECT COUNT(DISTINCT student_id) 
                               FROM   test_taken)AS rate 
FROM   test_taken 
WHERE  result < 50 
GROUP  BY test_id;

Answer 3

The OPs solution does not produce a correct result (at least not the one specified in the requirements) so expanding on that with UNION or other methods is NOT a valid answer. He seems to be calculating the percentages on the entire number of students instead of just the ones that took a specific test.

A query which would produce the correct result for each test is the following:

select Q1.test_id, 
   Q1.students_passed * 100 / Q1.total_students || '%' as pass_rate,
   Q1.students_failed * 100 / Q1.total_students || '%' as fail_rate
from       
(SELECT test_id, 
   sum(case when result > 50 then 1 else 0 end) students_passed,
   sum(case when result < 50 then 1 else 0 end) students_failed,
   count(distinct student_id) total_students
FROM   test_taken 
GROUP  BY test_id) Q1;

We first count how many students have passed and failed using the SUM function. We add one to the sum when our coditions are met, ie result > 50 or result < 50, else we add zero.

We also need to count the total nr. of students that have taken a test, so we can do that easily with count(distinct student_id) and grouping by test_id.

Finally we wrap this query in the outer one, in which we divide the the nr. of students passed and failed to the total nr of students for each a given test_id.

For the input table:

1  | 1 | 51
1  | 2 | 30
2  | 3 | 60
2  | 4 | 22
3  | 2 | 66

it produces the output:

1  | 50%  | 50%
2  | 50%  | 50%
3  | 100% | 0%