Inner Join with PHP - php

Question

I checked through a few different questions previously asked but they were more advanced than what I need at the moment. I need a simple way to join two tables and display the results so that I can then manipulate them in any way I want once it is collecting the data the way I need it to. The code below is very simple... Yet I am having trouble. First I create a class that connects to the database then I created a method to query the database and join to tables based on common columns. After that I would like the loop to go through the top four results based on their title name which are 'gold', 'silver', 'platinum', 'palladium' I just want to make sure that the join request is working. Please view the code below and maybe you can tell me why the results I keep getting are
1 Gold
1 Gold
1 Gold
1 Gold
Literally I get Gold 4 times when I need a list of all 4 precious metals.I thought that when the while loop runs through I would get each one as it is supposed to run through all 4 rows and there are no more yet it runs through the same 1st row and brings back 1 Gold every time. Both the id and the metals title name. If I am missing something please feel free to ask and I will add it for you if it helps.
class testJoin{
public function __construct($dbCon){
$this->dbConnection = $dbCon;
}
function testingJoin($dbCon) {
if($results = $this->dbConnection->query("SELECT metal.id, metal.title, price.metalId FROM metal INNER JOIN price ON metal.id = price.metalId ORDER BY metal.title LIMIT 0,4")){
while($data = $results->fetch_assoc()){
printf("<p style=\"display:inline;\">%s</p>
<p style=\"display:inline;\">%s</p><br />", $data['id'], $data['title']);
}
}
$dbCon->close();
}
}

Answer 1

JOIN creates a cross-product of the matching rows in the two tables. If there are multiple price rows for each metal, you'll get all those different prices, and then you take the first 4 rows of this.
If you want to limit the number of metals, but not the total number of rows, you can join with a subquery:
SELECT metal.id, metal.title, price.metalId
FROM (SELECT id, title
FROM metal
ORDER BY title
LIMIT 4) AS metal
JOIN price ON metal.id = price.metalId
Or if you want to get just one row per metal, you can use GROUP BY
SELECT metal.id, metal.title, price.metalId
FROM metal
JOIN price ON metal.id = price.metalId
GROUP BY metal.id
ORDER BY metal.title
LIMIT 4

Answer 2

Here you have no reason to join to prices table at all
SELECT metal.id, metal.title
FROM metal
ORDER BY metal.title
Cos u added to result nothing from there.
If u really need join to prices and display results by "not repeated" metal names, u should just GROUP results
SELECT metal.id, metal.title
FROM metal
INNER JOIN price ON (metal.id = price.metalId)
GROUP BY metal.id
ORDER BY metal.title
After that you can retrieve some useful data from prices table, for example average price for each metal
SELECT metal.id, metal.title, AVG(price.price) AS metal_price
FROM metal
INNER JOIN price ON (metal.id = price.metalId)
GROUP BY metal.id
ORDER BY metal.title
Also you should understand difference between LEFT JOIN and INNER JOIN.
LEFT - will fetch ALL needed rows from first table (metal) and add results from second (prices) even if there is no such metal in prices table (then results from second table will be NULL). (metal.id = price.metalId) can be understanded as "ALL metals with some prices, if they have"
INNER - will fetch ONLY those rows from first table which are presented in second table, by "JOIN ON" condition. (metal.id = price.metalId) can be understanded as "THOSE metals WHICH HAVE prices"
https://pp.vk.me/c623725/v623725696/14ae4/459rNGJwMJc.jpg