MySQL. Average price, connecting two databases - mysql

Question

I am learning MySQL and I currently do not understand how to do something.
I have two tables and I want to display some stuff out of it, it's pretty hard to explain so I'd rather show you.
These are my tables:
CREATE TABLE IF NOT EXISTS `proprietate` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`serie_buletin` varchar(8) NOT NULL,
`cnp` bigint(20) NOT NULL,
`nr_vehicul` int(11) NOT NULL,
`data_cumpararii` date NOT NULL,
`pret` int(11) NOT NULL,
`id_persoana` int(11) NOT NULL,
PRIMARY KEY (id),
) ENGINE=InnoDB DEFAULT CHARSET=latin1 AUTO_INCREMENT=16 ;
CREATE TABLE IF NOT EXISTS `vehicul` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`nr_vehicul` int(11) NOT NULL,
`marca` varchar(30) NOT NULL,
`id_marca` int(11) NOT NULL,
`tip` varchar(15) NOT NULL,
`culoare` varchar(15) NOT NULL,
`capacitate_cilindrica` int(11) NOT NULL,
`id_proprietate` int(11) NOT NULL,
PRIMARY KEY (id),
FOREIGN KEY (id_proprietate) REFERENCES proprietate(id)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 AUTO_INCREMENT=16 ;
And these are my values inside my tables:
INSERT INTO `proprietate` (`id`, `serie_buletin`, `cnp`, `nr_vehicul`, `data_cumpararii`, `pret`, `id_persoana`) VALUES
(1, 'AK162332', 2006036035087, 4, '2014-05-01', 35000, 1),
(2, 'AK162332', 2006036035087, 10, '2014-05-02', 90000, 2),
(3, 'AK176233', 6548751520125, 2, '2014-05-03', 55000, 3),
(4, 'BZ257743', 6548751520125, 2, '2014-05-04', 25000, 4),
(5, 'BZ257743', 2006036035087, 15, '2014-05-05', 63000, 5),
(6, 'DC456542', 2003564784513, 7, '2014-05-06', 30000, 6),
(7, 'EN654872', 2012654879521, 6, '2014-05-07', 50000, 7);
INSERT INTO `vehicul` (`id`, `nr_vehicul`, `marca`, `id_marca`, `tip`, `culoare`, `capacitate_cilindrica`, `id_proprietate`) VALUES
(1, 4, 'Mercedes', 1, 'CLK 350', 'negru', 3500, 1),
(2, 10, 'Mercedes', 1, 'S 500', 'silver', 5000, 2),
(3, 2, 'Mercedes', 1, 'ML 550', 'alb', 5500, 3),
(4, 2, 'BMW', 2, '325', 'galben', 2500, 4),
(5, 15, 'BMW', 2, 'X5', 'negru', 3500, 5),
(6, 7, 'Audi', 3, 'R5', 'mov', 5000, 6),
(7, 6, 'Audi', 3, 'Q5', 'metalic', 3000, 7);
What I want to display is:
marca | nr_vehicul | average_price
Audi | 13 | 40000
BMW | 17 | 44000
Mercedes | 16 | 60000
How can I do that? So far I have managed to display the first two columns but I have no idea how to reference the first table in the second and calculate the average price.
This is what I have so far:
SELECT marca, SUM(nr_vehicul) AS nr_vehicul FROM vehicul GROUP BY marca
Can anyone help me please?

Answer 1

You should join your tables to get combined information from both of them:
SELECT marca, SUM(vehicul.nr_vehicul) AS nr_vehicul, avg(pret) as pret
FROM vehicul
LEFT OUTER JOIN proprietate on (id_proprietate = proprietate.id)
GROUP BY marca;
see this sql fiddle session for the output.
First you select the data (column names with appropriate functions used) you need: marca, SUM(vehicul.nr_vehicul), AVG(pret), then you construct the joined structure from where mysql should retrieve these informations: vehicul, proprietate.
For this structure you need primarily the vehicul table, by which you will group the result set. You want to join the proprietate table to the vehicul table properly, to make sure the correct data structure is created. Since you have foreign key from one table to the other, the easiest way to do it is to use that key: LEFT OUTER JOIN proprietate on (id_proprietate = proprietate.id).
For more information on understanding the different JOIN types, please see this article by Craig Buckler.

Answer 2

$query = "SELECT type, AVG(pret) FROM vehicul GROUP BY marca";
$result = mysql_query($query) or die(mysql_error());
// Print out result
while($row = mysql_fetch_array($result)){
echo "The average price of ". $row['type']. " is $".$row['AVG(price)'];}</code>
should return the average price per marca