MySQL如何同时连接到多个数据库？

我的本地主机上有多个使用相同用户名和密码的数据库。由于信息存储在不同的数据库中，我需要跨数据库工作，同时打开两个数据库。我将数据插入到一个 DB 中，并希望删除另一个 DB 中的表以及数据。

这是我目前正在尝试的连接方式：

$host = "localhost"; // Host name 
$username = "root"; // Mysql username 
$password = "@"; // Mysql password 
$db_name = "etrack"; // Database 1
$db_name2 = "gs"; // Database 2

// Create connection
$conn = new mysqli($host, $username, $password, $db_name);
$conn2 = new mysqli($host, $username, $password, $db_name2);
// Check connection
if ($conn2->connect_error) {
    die("Connection failed: " . $conn2->connect_error);

}

然后我运行了一些SQL字符串，其中一个例子是：

$sql = "INSERT INTO unit_hx SET
                imei = '".$imei."',
                model = '".$model."',
                supp = '".$supp."',
                status = 'Scrapped',
                installer = '".$installer."',
                unit_nr = '".$unit_nr."',
                cost = '".$cost."',
                systime = '".$time."',
                msisdn = '".$msisdn."',
                sysdate = '".$date."',
                notes = '".$notes."',
                staff = '".$agent."'";

我想根据在数据库eTrack中检索到的IMEI信息在数据库GS中删除表：

DROP TABLE 'gs_object_data_'".$imei."' 
//imei information is set on the page from a previous form using $_POST

用DB1中的信息是否可能查询DB2和删除DB2的表？

更新工作代码：要在两个不同的数据库中插入或更新信息，以下代码非常有用：

    "INSERT INTO etrack.simcard_hx SET" //etrack is the database name simcard_hx is the table name
Changing database to other database I used 
"INSERT INTO gs.simcard_hx SET" // gs being the second database

Your root user should have access to both databases, so you shouldn't need the second connection. If it doesn't, create a user that does, it will simplify your life. Then, in queries you can just prefix your table names with the database name: e.g.

SELECT * 
FROM etrack.table1 t1
JOIN gs.table2 t2 ON t2.etrack_id = t1.id