Using two columns with the same foreign key - mysql

Question

I have a database, which has 5 tables, but I have problem. In the shipment table, there are two columns, source and destination, which are both foreign key references to the route table, but when I select a record from the shipment table it will only show the same routename for both.
Here is the code:
SELECT fare, commission, driver,shipment._date, routename, vehiclenumber, productname, source, destination, routename,ownername
FROM route, shipment, product, vehicle,owner
WHERE vehicle.vehicleid = shipment.vehicle
AND shipment.source
and vehicle.owner=owner.ownerid
AND shipment.destination = route.routeid
AND shipment.product = product.productid
AND vehicle.vehiclenumber = 'nk-234'
ORDER BY _date
LIMIT 0 , 30

Answer 1

To connect a record in one table (one shipment) to more than one record in another table (two routes) you would ideally use explicit JOINs against the route table twice (or more, for however many links you happen to need).
Here's a quick modification to your query that demonstrates usage. Pay particular attention to the two routename columns in the SELECT and the two JOINs to the route table in the FROM:
SELECT fare,
commission,
driver,
shipment._date,
RS.routename, <-- field from first join
vehiclenumber,
productname,
source,
destination,
RD.routename, <-- field from second join
ownername
FROM shipment
JOIN route RS ON RS.routeID = shipment.source <-- join 1; source
JOIN route RD ON RD.routeID = shipment.destination, <-- join 2; destination
product,
vehicle,
owner
WHERE vehicle.vehicleid = shipment.vehicle
AND vehicle.owner = owner.ownerid
AND shipment.product = product.productid
AND vehicle.vehiclenumber = 'nk-234'
ORDER BY _date
LIMIT 0 , 30
Obviously, this might not match your needs exactly, because I don't know what tables the other selected fields are from to be able to make sure they are all accounted for...

Answer 2

This is because you select the fields from one row in shipment, in orer to actually get any effect from your foreign keys you need to use a few joins
http://dev.mysql.com/doc/refman/5.5/en/join.html

Answer 3

Using the old-style (non-preferred) notation, you can write:
SELECT s.fare, s.commission, s.driver, s._date, v.vehiclenumber, p.productname,
s.source, s.destination, rs.routename, rd.routename, o.ownername
FROM route AS rs, route AS rd, shipment AS s, product AS p, vehicle AS v, owner AS o
WHERE s.vehicleid = s.vehicle
AND s.source = rs.routeid
AND v.owner = o.ownerid
AND s.destination = rd.routeid
AND s.product = p.productid
AND v.vehiclenumber = 'nk-234'
ORDER BY _date
LIMIT 0, 30
I had to guess which table(s) contain the fare, commission and driver information; I assumed they are all in the shipment table.
Starting a column name with an underscore is not strictly allowed in standard SQL, though most DBMS do allow it. It looks ugly, though; why not 'ship_date' as the column name?
It is better to use the explicit join notation, not least because there is less chance of forgetting a join condition:
SELECT s.fare, s.commission, s.driver, s._date, v.vehiclenumber, p.productname,
s.source, s.destination, rs.routename, rd.routename, o.ownername
FROM shipment AS s
JOIN route AS rs ON s.source = rs.routeid
JOIN route AS rd ON s.destination = rs.routeid
JOIN vehicle AS v ON s.vehicleid = s.vehicle
JOIN owner AS o ON v.owner = o.ownerid
JOIN product AS p ON s.product = p.productid
WHERE v.vehiclenumber = 'nk-234'
ORDER BY s._date
LIMIT 0, 30