MySQL Query: 3 tests passed, 1 failed - mysql

Question

I'm trying to write a query that will pull students that have passed tests 1 thru 3 AND failed test 4.
Students can retake tests so there may be failed records, followed by passed records for some tests, such is the case with student_id = 2 below.
Table setup like this -
test_id | student_id | status | completed_on
--------+------------+---------+------------
1 | 1 | passed | 2018-03-24
2 | 1 | passed | 2018-03-25
3 | 1 | passed | 2018-03-26
4 | 1 | failed | 2018-03-27
1 | 2 | failed | 2018-03-24
1 | 2 | passed | 2018-03-25
2 | 2 | passed | 2018-03-26
3 | 2 | passed | 2018-03-27
4 | 2 | failed | 2018-03-27
In this case the query should pull both student_id 1 and 2
I tried this but it obviously didn't work -
select *
from table
where (test_id = 1 and status = 'passed')
and (test_id = 2 and status = 'passed')
and (test_id = 3 and status = 'passed')
and (test_id = 4 and status = 'failed')

Answer 1

Demo
SELECT count(Z.Test_ID), Z.student_ID
FROM (SELECT distinct student_ID, test_ID, Status
FROM table) Z
WHERE (Z.Status = 'Passed' and Z.test_ID in (1,2,3,4))
OR (Z.status = 'Failed' and Z.test_ID = 4)
GROUP BY Z.Student_ID
HAVING count(Z.Test_ID) = 4;
This works by first ensuring we only have distinct records for each student, status, and test_ID. (derived table Z)
We then evaluate how many passes in tests 1,2,3,4 exist and existence of fails we have for test 4. if the count is anything other than 4 then we know either they didn't pass the tests 1-3 and fail 4 or they've passed test 4 as well.

Answer 2

I'm not claiming this is fast or most efficient, but it will do the job. Make sure you have the right indices on your table,
SELECT s1.student_id
FROM mytable s1
JOIN mytable s2 on s1.student_id=s2.student_id and s2.test_id=2 and s2.status='passed'
JOIN mytable s3 on s1.student_id=s3.student_id and s3.test_id=3 and s3.status='passed'
WHERE s1.test_id=1
AND s1.status='passed'
AND NOT EXISTS (
SELECT 1
FROM mytable s4
WHERE s4.student_id=s1.student_id
AND s4.test_id=4
AND s4.status='passed'
)

Answer 3

Another approach:
select distinct t1.student_id
from mytable t1
inner join
-- students passed all the 3 tests
(select student_id from mytable where test_id in (1, 2, 3)
and status = 'passed' group by student_id having count(distinct test_id) = 3 ) t2
on t1.test_id = 4 and t1.status = 'failed' and t1.student_id = t2.student_id
where not exists
(select 1 from mytable where student_id = t1.student_id and
status = 'passed' and test_id = 4)
PS. If a student passed a test (e.g. test 1) but later took it again and failed, the student would be considered as passed. Not sure if that is acceptable.

Answer 4

One approach you can use to meet your requirement is to basically make a pivot table. Any way you slice it you will probable need to use a subquery, here using the SUM function and CASE statements you can determine how many times each student passed a test.
Then, in your outer WHERE clause you can select only the rows where you got 1 or more 'passed' results for tests 1 through 3, and 0 'passed' results for test 4.
SQL Fiddle
SELECT student_id
FROM (SELECT student_id,
SUM(
CASE WHEN(test_id = 1 AND result = 'passed')
THEN 1
ELSE 0
END) AS "Test1",
SUM(
CASE WHEN(test_id = 2 AND result = 'passed')
THEN 1
ELSE 0
END) AS "Test2",
SUM(
CASE WHEN(test_id = 3 AND result = 'passed')
THEN 1
ELSE 0
END) AS "Test3",
SUM(
CASE WHEN(test_id = 4 AND result = 'passed')
THEN 1
ELSE 0
END) AS "Test4"
FROM TestResults
GROUP BY student_id) tr
WHERE Test1 > 0 AND Test2 > 0 AND Test3 > 0 AND Test4 = 0
Using this technique you can also determine how many times a student passed or failed a test. So for example, you can change the CASE statements to 'failed' and return the test columns in the outer query to see the number of times a student failed the test. For example:
SELECT student_id, Test1Fails
FROM (SELECT student_id,
SUM(
CASE WHEN(test_id = 1 AND result = 'failed')
THEN 1
ELSE 0
END) AS "Test1Fails"
FROM TestResults
GROUP BY student_id) tr