MySQL query across 2 databases with different users - mysql

Question

I have a mysql query that joins data across 2 databases and inserts new records into one of the databases. The query below works if the same mysql user has access to both databases and both are on the same server. However, how would you re-write if each database has different user credentials?
PHP Script snippet:
$hard = mysql_connect($hostname_hard, $username_hard, $password_hard)
or trigger_error(mysql_error(),E_USER_ERROR);
# Insert artists:
mysql_select_db($database_hard, $hard);
$query_artsync = "insert ignore into {$joomla_db}.jos_muscol_artists
(artist_name, letter, added,url,class_name)
select distinct
artist
, left(artist,1) as letter
, now()
, website, artist
from {$sam_db}.songlist
where (songtype='s' AND artist <>\"\")";
mysql_query($query_artsync, $hard) or die(mysql_error());
echo "<br>Artist tables merged! <br><br> Now merging albums<br><br>";
So..in the above the {$sam_db} database is accessed by a different user than the {$joomla_db} user...
There are more complex inserts following this one, but I think if I can get the above to work, then I'll likely be able to apply the same principles to the other insert queries...

Answer 1

you're talking about using 2 different connections in the same query, which, unfortunately, is not possible. what you'll have to do is get (select) all the information you need from the one database, and use that info to construct your insert query on the other database (2 separate queries).
Something like this:
$result = mysql_query("SELECT DISTINCT artist, LEFT(artist,1) AS letter, now() as added, website
FROM {$sam_db}.songlist
WHERE (songtype='s' AND artist <>\"\"", $sam_con);
while(($row = mysql_fetch_assoc($result)) != false)
{
$artist = mysql_real_escape_string($row['artist']);
$letter = mysql_real_escape_string($row['letter']);
$added = mysql_real_escape_string($row['added']);
$website = mysql_real_escape_string($row['website']);
mysql_query("INSERT IGNORE INTO {$joomla_db}.jos_muscol_artists
(artist_name, letter, added,url,class_name)
VALUES
('$artist', '$letter', '$added', '$website', '$artist')", $joomla_con);
}
where $sam_con and $joomla_con are your connection resources.

Answer 2

There is no problem querying tables from different databases.
SELECT a.*, b.*
FROM db1.table1 a
INNER JOIN db2.table2 b ON (a.id = b.id)
Will run with no problems, as will your insert query.
However the user that starts the query needs to have proper access to the databases and tables involved.
That means that user1 (who does the insert) has to be granted insert+select rights to table {$joomla_db}.jos_muscol_artists and select rights to {$sam_db}.songlist
If you don't want to expand the rights of your existing users, then you can just create a new inserter user that has the proper access rights to use both databases in the proper manner.
Only use this user to do inserts.
Alternative solution without adding users
Another option is to create a blackhole table on db1 (the db you select from)
CREATE TABLE db1.bh_insert_into_db2 (
f1 integer,
f2 varchar(255),
f3 varchar(255)
) ENGINE = BLACKHOLE;
And attach a trigger to that that does the insert into db2.
DELIMITER $$
CREATE TRIGGER ai_bh_insert_into_db2_each ON bh_insert_into_db2 FOR EACH ROW
BEGIN
INSERT INTO db2.table2 (f1,f2,f3) VALUES (NEW.f1, NEW.f2, NEW.f3);
END $$
DELIMITER ;
The insert into table db2.table2 will happen with the access rights of the user how created the trigger.
http://dev.mysql.com/doc/refman/5.1/en/grant.html

Answer 3

With PHP for example, you will have to create 2 unique mysql instances, 1 per connection.
Then use both and do individual queries.
Explanation...
Setup connections to both databases using mysql_connect for example. Now, you now have defined the connenction variables...
In the mysql_query you apply those variables, e.g mysql_query($query, $connect1) or mysql_query($query, $connect2).
From there you can extract and insert using code.