Tricky SQL query involving consecutive values
- by Gabriel
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 | 2012-01-04
Ann | Evans | 2012-02-03
How can I perform such a query?
Thank you for your time.
