How do I construct a cross database query in PHP?

Question

In our last episode (How I constructed a cross database query in MySQL) I learned how to construct a cross database query in MySQL. This worked great but when our hero tried to use this newfound knowledge in PHP he found his best friend FAIL waiting for him.

I took a look at mysql_select_db for PHP. This seems to imply that if I want to use MySQL with PHP, I have a couple of choices:

1. Use mysql_select_db but be stuck with only using one db at a time. This is our current setup and putting a database as a namespace identifier doesn't seem to work (it works fine in the MySQL shell so I know it's not a problem with our MySQL server setup).

2. Don't use mysql_select_db. From some of the examples I've seen, this seems to mean that I have to specify the db for every query that I make. This makes sense since I haven't used mysql_select_db to tell PHP what db I want to access. This also makes sad since I don't want to go through all my code and prepend a db name to every query.

Is there something better than this? Is there a way for me to do a cross db MySQL query in PHP without having to something crazy like (2)?

CLARIFICATION: None of the proposed answers actually let me do a cross db query. Instead, they allow me to access two different DBs separately. I want a solution that allows me to do something like SELECT foreign_db.login.username, firstname, lastname from foreign_db.login, user where ... NOT just make different queries to different DBs. For what it's worth, (2) doesn't work for me.

Answer 1

You will need your databases to run on the same host.

If so, you should be able to use mysql_select_db on your favourite/default db and manually specify a foreign database.

$db = mysql_connect($hots, $user, $password);
mysql_select_db('my_most_used_db', $db);

$q = mysql_query("
    SELECT *
    FROM   table_on_default_db a, `another_db`.`table_on_another_db` b
    WHERE  a.id = b.fk_id
");

If your databases run on a different host, you won't be able to join directly. But you can then make 2 queries.

$db1 = mysql_connect($host1, $user1, $password1);
$db2 = mysql_connect($host2, $user2, $password2);

$q1 = mysql_query("
    SELECT id
    FROM   table
    WHERE  [..your criteria for db1 here..]
", $db1);
$tmp = array();
while($val = mysql_fetch_array($q1))
    $tmp[] = $val['id'];

$q2 = mysql_query("
    SELECT *
    FROM   table2
    WHERE  fk_id in (".implode(', ', $tmp).")
", $db2);

Answer 2

After reading your clarification, I am under the impression that you actually want to query tables residing in two separate MySQL server instances. At least, your clarification text:

SELECT foreign_db.login.username, firstname, lastname from foreign_db.login, user where

suggests that you want to run one query while being logged in as two users (which may or may not reside on the same mysql server instance).

In your question, you said you wanted to query data from two different databases, but it is important to realize that one MySQL instance can have many, many databases. For multiple databases managed by the same mysql instance, the solution proposed in the question you linked to simply works: just prefix the table name with the name of the databases, separating database and table names with a dot: <db-name>.<table-name>.

But, like i pointed out, this only works if:

1. all databases you access in one query reside on the same server - that is, are managed by the same MySQL instance
2. the user that is connected to the database has the right privileges to access both tables.

Scenario1: databases on same host: grant appopriate privileges and qualify table names

So if the tables actually reside on the same mysql instance, there is no need for a second login or connection - simply grant the database user you use to connect to the datbase the appropriate privileges to select from all tables you need. You can do that with the GRANT syntax, documented here: http://dev.mysql.com/doc/refman/5.1/en/grant.html

For example, GRANT SELECT ON sakila.film TO 'test'@'%' will allow the user test@% to select data from the film table in the sakila database. After doing that, said user can refer to this table using sakila.film (so-called qualified table name), or if the current database is set to sakila, simply as film

Scenario2: databases managed by different MySQL instances: FEDERATED engine

If the tables you want to access are actually managed by two different MySQL instances, there is one trick that may or may not work, depending on your configuration. Since MySQL 5.0 mysql supports the FEDERATED storage engine. This lets you create a table that is not actually a table, but a peephole to a table on a remote server. This engine is documented here: http://dev.mysql.com/doc/refman/5.1/en/federated-storage-engine.html

For example, if you know there is this table in the misc database on the remote host:

CREATE TABLE t (
    id int not null primary key
,   name varchar(10) not null unique
)

you can make a local 'pointer' to that remote table using this:

CREATE TABLE t (
    id int not null primary key
,   name varchar(10) not null unique
)
ENGINE = FEDERATED
CONNECTION='mysql://<user>@<remote-server>:<remote-port>/misc/t';

Unfortunately, the FEDERATED engine is not always available, so you have to check first if you can even use that. But suppose it is, then you can simply use the local table t in your queries, just like any other table, and MySQL will communicate with the remote server and perform the appropriate operations on the physical table on the other side.

Caveat: there are several optimization issues with FEDERATED tables. You should find out if and to what extent these apply to you. For instance, applying a WHERE to a federated table can in many cases result in the entire table contents being pullled over the wire to your local server, where the actual filtering will be appplied. Another issue is with table creation: you have to be very sure that the definitions of the federated table and the table it is pointing to match exacty, except for the ENGINE clause (and CONNECTION). If you have for example a different character set, the data may arrive completely garbled after travelling over the wire.

If you want to use FEDERATED tables, do read this article http://oreilly.com/pub/a/databases/2006/08/10/mysql-federated-tables.html to decide if its right for your particular use case.

If you think you do need it, I have a utility to create federated tables here: http://forge.mysql.com/tools/tool.php?id=54

Scenario3: can't use FEDERATED, but tables on different MySQL instances

Finally, if you have tables on different MySQL instances, but cannot for some reason use the federated table engine, your a out of luck I'm afraid. You are simply going to have to execute queries to both MySQL instances, receive the results and do something intelligent with it in PHP. depending on your exact requirements, this may be a perfectly viable solution

I guess you need to decide for yourself which part of my answer best appeals to your problem, and add a comment in case you need more help. TIA Roland.