SQL on how to simultaneously compare two column values in a query - sql

Question

I need create a query to find the names of drivers whose exam scores get lower when they take more exams. So I have the following tables:
branch(branch_id, branch_name, branch_addr, branch_city, branch_phone);
driver(driver_ssn, driver_name, driver_addr, driver_city, driver_birthdate, driver_phone);
license(license_no, driver_ssn, license_type, license_class, license_expiry, issue_date, branch_id);
exam(driver_ssn, branch_id, exam_date, exam_type, exam_score);
**the exam_date is a date
So I am using the tables driver and exam. I would like to somehow check that the exam_date>anotherDate while at the same time checking that exam_score
*EDIT
This is what I came up with but I feel like some of the syntax is illegal. I keep getting a syntax error.
s.executeQuery("SELECT driver_name " +
"FROM driver " +
"WHERE driver.driver_ssn IN " +
"(SELECT e1.driver_ssn" +
"FROM exam e1" +
"WHERE e1.exam_score < " +
"(SELECT e2.exam_score FROM exam e2)" +
"AND e1.exam_date > " +
"(SELECT e2.exam_date FROM exam e2)");
EDIT! I got it working! Thanks for your input everyone!
SELECT driver.driver_name
FROM driver
WHERE driver.driver_ssn IN
(SELECT e1.driver_ssn
FROM exam e1, exam e2, driver d
WHERE e1.exam_score < e2.exam_score
AND e1.exam_date > e2.exam_date
AND e1.driver_ssn=e2.driver_ssn)

Answer 1

You will need to do a self join. See this example and work it to your schema.
select d.name,
es.date_taken as 'prev date',
es.score as 'prev score',
es.date_taken as 'new date',
es_newer.score as 'new score'
from driver d
inner join exam_score es
on d.id = es.driver_id
left outer join exam_score es_newer
on d.id = es_newer.driver_id
and es_newer.date_taken > es.date_taken
and es_newer.score < es.score
where es_newer.id is not null
Here is a SQL Fiddle that I made to demonstrate.

Answer 2

SELECT returns a set and you cannot compare a single value against a set. You can try something along these lines. This is similar to yours and doesn't handle three exam case :-
SELECT driver_name
FROM driver
JOIN exam e1 ON driver_ssn
JOIN exam e2 ON driver_ssn
WHERE e1.exam_score < e2.exam_score
AND e1.exam_date > e2.exam_date
The query selects all pairs of exams taken by a driver in which the score is less and the date big

Answer 3

Simple take on this problem would be getting drivers who take a couple of exams and their second score is lower.
To compare columns from the same table SQL uses self-join. Your join condition should include:
select e1.driver_ssn, e1.exam_type, e1.exam_score as score_before,
e2.exam_score as score_after
exam e1 join exam e2 on (e1.driver_ssn = e2.driver_ssn and
e1.exam_type = e2.exam_type and
e1.exam_date < e2.exam_date and
e1.exam_score > e2.exam_score)