Question

There is a table employee in the database abc_db at abc@localhost(server) and there is another table department in the database xyz_db at xyz@localhost(server). How can I join the tables using php mysql connection. I have written the following code but it does not generate any resource id.

$conn = mysql_connect("localhost","abc","abcdb");
$conn1 = mysql_connect("localhost","xyz","xyzdb");

$db_select=mysql_select_db("abc_db",$conn);
$db_select1=mysql_select_db("xyz_db",$conn1);

$sql="SELECT * FROM employee e LEFT JOIN department d ON e.department_id=d.id ";
$res=mysql_query($sql);

Answer 1

You can’t join two tables using different connections to the database, not from PHP, nor on the MySQL server. (@RobertPitt has a good point: do you actually need two connections? It’s possible to join two tables on the same host, but in different databases, within one connection – assuming your connection has the necessary privileges to access both)

If you have control over one or other of the databases, you might try setting up a federated table; make sure that the performance is OK though (if the db machines don’t have a fast, low-latency connection (i.e. directly joined by a cable), don’t bother), and there is a long list of limitations.

Possible lesser evils:

• replicate the table from one server to the other (tricky to set up)
• “join” them manually in PHP (gross, inefficient, but pretty much your only choice if you don’t have control over the database)