PostgreSQL questions, constraints and queries - sql

Question

My task is to make a table that records the placement won by race car drivers competing in Race events.
The given schema is:
CREATE TABLE RaceEvent
(
Name text,
Year int,
);
CREATE TABLE Driver
(
Name text,
Date_of_birth date,
Gender char,
Nationality,
);
I then added the following constraints :
CREATE TABLE RaceEvent
(
RaceName text NOT NULL PRIMARY KEY,
Year int NOT NULL,
Description text NOT NULL
);
CREATE TABLE Driver
(
Name text NOT NULL,
Date_of_birth date NOT NULL PRIMARY KEY,
Gender char(1) NOT NULL,
Nationality text NOT NULL
);
The table I created looks like this :
CREATE TABLE Races
(
Medal char(6) CHECK (Medal = 'Gold' or Medal = 'Silver' or Medal =
'Bronze'),
Event text NOT NULL REFERENCES RaceEvent (Name),
DriverDOB date NOT NULL REFERENCES Driver (Date_of_birth)
);
I know using the date of birth as a primary key is very silly but for some reason that was part of the task.
I need to ensure a driver cannot gain multiple medals in the same race, can anybody give insight on a good way of doing this? I thought about using some sort of check but can't quite work it out.
After that, I need to write a query that can return the nationalities of drivers that won at least 2 gold medals in certain years, to figure out which nationalities seem to produce the best drivers. 2 versions of the same query, one using aggregation and one not.
I know I have to do something along these lines :
SELECT Nationality from Driver JOIN Races ON Driver.Date_of_Birth = Races.DriverDOB WHERE ....?
Not sure on what the best way of figuring out how to link the nationalities to the medals?
All feedback much appreciated

Answer 1

The "best" way to do it would be to restructure your schema, as right now it's pretty crap. I'm assuming you can't, so here's one way to prevent multiple drivers from gaining multiple medals in the same race: add a primary key on DriverDOB and Event to the Races table.
Try it out here: http://sqlfiddle.com/#!17/dc8a9/1
As for the query to get the nationalities with multiple golds in a given year, here's one way to do it:
SELECT d.nationality, COUNT(*) AS golds
FROM races r
JOIN driver d
ON r.driverdob = d.date_of_birth
JOIN raceevent e
ON r.event = e.racename
AND e.year = 1999
WHERE r.medal = 'Gold'
GROUP BY d.nationality
HAVING COUNT(*) > 1;
Output:
nationality golds
NatA 3
NatB 2
And you can test it here: http://sqlfiddle.com/#!17/dc8a9/9