Tricky SQL query involving consecutive values - mysql

Question

I need to perform a relatively easy to explain but (given my somewhat limited skills) hard to write SQL query.
Assume we have a table similar to this one:
exam_no | name | surname | result | date
---------+------+---------+--------+------------
1 | John | Doe | PASS | 2012-01-01
1 | Ryan | Smith | FAIL | 2012-01-02 <--
1 | Ann | Evans | PASS | 2012-01-03
1 | Mary | Lee | FAIL | 2012-01-04
... | ... | ... | ... | ...
2 | John | Doe | FAIL | 2012-02-01 <--
2 | Ryan | Smith | FAIL | 2012-02-02
2 | Ann | Evans | FAIL | 2012-02-03
2 | Mary | Lee | PASS | 2012-02-04
... | ... | ... | ... | ...
3 | John | Doe | FAIL | 2012-03-01
3 | Ryan | Smith | FAIL | 2012-03-02
3 | Ann | Evans | PASS | 2012-03-03
3 | Mary | Lee | FAIL | 2012-03-04 <--
Note that exam_no and date aren't necessarily related as one might expect from the kind of example I chose.
Now, the query that I need to do is as follows:
From the latest exam (exam_no = 3) find all the students that have failed (John Doe, Ryan Smith and Mary Lee).
For each of these students find the date of the first of the batch of consecutively failing exams. Another way to put it would be: for each of these students find the date of the first failing exam that comes after their last passing exam. (Look at the arrows in the table).
The resulting table should be something like this:
name | surname | date_since_failing
------+---------+--------------------
John | Doe | 2012-02-01
Ryan | Smith | 2012-01-02
Mary | Lee | 2012-03-04
How can I perform such a query?
Thank you for your time.

Answer 1

You can take advantage of the fact that if someone passed the most recent exam, then they have not failed any exams since their most recent pass: therefore the problem reduces to finding the first exam failed since the most recent pass:
SELECT name, surname, MIN(date) date_since_fail
FROM results NATURAL LEFT JOIN (
SELECT name, surname, MAX(date) lastpass
FROM results
WHERE result = 'PASS'
GROUP BY name, surname
) t
WHERE result = 'FAIL' AND date > IFNULL(lastpass,0)
GROUP BY name, surname
See it on sqlfiddle.

Answer 2

I should use a subquery that fetch last passed exam,
somthing like:
SET #query_exam_no = 3;
SELECT
name,
surname,
MIN(IF(date > last_passed_exam, date, NULL)) AS date_failing_since
FROM
exam_results
LEFT JOIN (
SELECT
name,
surname,
MAX(date) AS last_passed_exam
FROM exam_results
WHERE result = 'PASS'
GROUP BY name, surname
) AS last_passed_exams USING (name, surname)
HAVING
MAX(IF(exam_no = #query_exam_no, result, NULL)) = 'FAIL'
GROUP BY name, surname

Answer 3

This is enough:
select t.name,
t.surname,
t.date as 'date_since_failing'
from tablename t
inner join
(
select name,
surname,
max(exam_no) as exam_no
from tablename
group by name, surname
having min(result) = 'FAIL'
) aux on t.name = aux.name and t.surname = aux.surname and t.exam_no = aux.exam_no

Answer 4

The condition you are asking for is good for nothing you can do it without it. Here is the working example.
select
e.name,
e.sur_name,
min(e.date) as `LastFailed`
from exams as e
where e.result = 'Fail'
group by e.name
order by e.name
This produces this result
name sur_name LastFailed
Ann Evans 2012-02-03
John Doe 2012-02-01
Mary Lee 2012-01-04
Ryan Smith 2012-01-02