Joining two tables from different databases - php

Question

I am aware that there are multiple posts about this, but I was not able to make it work for my code.
As the title suggests I want to join two tables from two different DBs together.
Here is my code:
$dbh1 = mysql_connect("$host", "$username", "$password")or die("cannot connect");
$dbh2 = mysql_connect("$host2", "$username2", "$password2", true)or die("cannot connect");
mysql_select_db("$db_name", $dbh1)or die("cannot select DB");
mysql_select_db("$db_name2", $dbh2)or die("cannot select DB");
//first table
//$sql = mysql_query("SELECT InterestedEntityId, Score FROM users.`user_interests` WHERE UserId= //$userID ORDER BY Score DESC", $dbh1);
//second table
//$sql = mysql_query("SELECT entities.`Name` FROM tags.`entities` WHERE Id = InterestedEntityId", $dbh2);
I want to get the 3 fields mentioned in select statements in one go (I.E. InterestedEntityId, Score, entities.Name)
Any idea on how to join these two tables in one sql query. I tried using inner joins and adding the tablename (as this thread suggested), but the query did not return anything.
Any ideas please?

Answer 1

Something like this should work.
SELECT t1.InterestedEntityId, t1.Score, t2.Name
FROM DB1.users.`user_interests` t1
JOIN DB2.tags.`entities` t2 ON t2.UserId = t1.Id
Note: Use PDO as mysql_* is deprecated and not secure enough.

Answer 2

Pretty much the model is:
SELECT dbName1.TableName1.ColumnName1, dbName2.TableName2.ColumnName2 FROM dbName1.TableName1 JOIN dbName2.TableName2 ON dbName1.TableName1.ColumnName1 = dbName2.TableName2.ColumnName2

Answer 3

Please, don't use mysql_* functions in new code. They are no longer maintained and are officially deprecated. See the red box? Learn about prepared statements instead, and use PDO or MySQLi - this article will help you decide which. If you choose PDO, here is a good tutorial.
While it is theoretically possible to join tables from two different databases on the same server, what you are trying to do cannot possibly work because you appear to be accessing two different servers.
In order to get the result set you want you will need to combine them manually.
For example (using PDO):
$dsn1 = "mysql:host=$host;dbname=$db_name";
$dsn2 = "mysql:host=$host2;dbname=$db_name2";
try {
// Create the connections
$db1 = new PDO($dsn1, $username, $password);
$db1->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
$db1->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);
$db1->setAttribute(PDO::ATTR_DEFAULT_FETCH_MODE, PDO::FETCH_ASSOC);
$db2 = new PDO($dsn2, $username2, $password2);
$db2->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
$db2->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);
$db2->setAttribute(PDO::ATTR_DEFAULT_FETCH_MODE, PDO::FETCH_ASSOC);
// Get the initial recordset
$sql1 = "
SELECT InterestedEntityId, Score
FROM `user_interests`
WHERE UserId = :userId
ORDER BY Score DESC
";
$stmt1 = $db1->prepare($sql1);
$stmt1->bindParam('userId', $userID, PDO::PARAM_INT);
$stmt1->execute();
// Prepare the statement for the second database
$sql2 = "
SELECT Name
FROM entities
WHERE Id = :entityId
";
$entityId = 0;
$stmt2 = $db2->prepare($sql2);
$stmt2->bindParam('id', $entityId, PDO::PARAM_INT);
// Loop the first result set
$result = array();
foreach ($stmt1 as $row1) {
// Fetch the related data from the second DB
$entityId = $row1['InterestedEntityId'];
$stmt2->execute();
$row2 = $stmt2->fetch();
// Construct the final result row and store it
$result[] = array(
'InterestedEntityId' => $row1['InterestedEntityId'],
'Score' => $row1['Score'],
'Name' => $row2['Name']
);
}
} catch(PDOException $e) {
die($e->getMessage());
}
// The result set you want should now be available
var_dump($result);