Moving rows from one database to another using mysqli - php

Question

My question is pretty straight-forward, and I assume that people need to do this pretty often; yet, after hours of searching online, nothing especially enlightening has come up. I am aware of a thread on here about doing the same thing using PDO, but I want to stick to mysqli to be consistent across my whole site.
So, I need to move one or several rows from one database to another (and NOT from one table to another within a single database).
The code I have so far is sure to get some laughs, as this task proved to be considerably beyond my ability. Anyway, here goes:
To begin with, I have two mysqli_connect... not sure if that is encouraged, although it has not caused me problems so far.
$connect_MAIN = mysqli_connect($servername_MAIN, $username_MAIN, $password_MAIN, $dbname_MAIN);
$connect_TEMP = mysqli_connect($servername_TEMP, $username_TEMP, $password_TEMP, $dbname_TEMP);
Here is the meat in the sandwich in terms of code:
///Prepare this in advance because the 'IN' values are taken from a $_GET
$sql_select = "SELECT * FROM Concert WHERE id IN (1, 2, 3, 4)";
///Connect to source DB
$result = $connect_TEMP->query($sql_select);
if ($result->num_rows != 0) {
///Connect to destination DB
$stmt = $connect_MAIN->prepare('INSERT INTO Concert (venue_id, date, ensemble_id, info, title, repertoire, time) VALUES (?, ?, ?, ?, ?, ?, ?)');
$venue_id = $date = $ensemble_id = $info = $title = $repertoire = $time = null;
$stmt->bind_param("isissss", $venue_id, $date, $ensemble_id, $info, $title, $repertoire, $time);
while($row = $result->fetch_assoc()) {
$venue_id = $row["venue_id"];
$date = $row["date"];
$ensemble_id = $row["ensemble_id"];
$info = $row["info"];
$title = $row["title"];
$repertoire = $row["repertoire"];
$time = $row["time"];
if ($stmt->execute() === TRUE) {
if ($show_once == 1) {
echo "Info successfully submitted.";
$show_once = 0;
}
} else {
echo "Hmm, something went wrong..." . $connect_MAIN->error;
}
}
}
$stmt->close();
mysqli_close($connect_TEMP);
}
So, I know that this is a bit of a jumble... as I said, I am not sure if it is possible to connect to DB2 in the middle of a while loop returning values from DB1. Feel free to mock, but only if you have something useful to suggest!

Answer 1

Use a parametrized query instead of concatenating strings.
///Prepare this in advance because the 'IN' values are taken from a $_GET
$sql_select = "SELECT * FROM Concert WHERE id IN (1, 2, 3, 4)";
///Connect to source DB
$result = $connect_TEMP->query($sql_select);
if ($result->num_rows != 0) {
///Connect to destination DB
$stmt = $connect_MAIN->prepare('INSERT INTO Concert (venue, date, info, time) VALUES (?, ?, ?, ?)');
$venue = $date = $info = $time = null;
$stmt->bind_param("ssss", $venue, $date, $info, $time);
while($row = $result->fetch_assoc()) {
$venue = $row["venue"];
$date = $row["date"];
$info = $row["info"];
$time = $row["time"];
if ($stmt->execute()) {
///This is just to stop message from appearing multiple times.
if ($show_once == 1) {
echo "Info successfully submitted.";
$show_once = 0;
}
} else {
echo "Hmm, something went wrong..." . $stmt->error;
}
}
$stmt->close();
mysqli_close($connect_TEMP);
}

Answer 2

Yes, you may establish a second DB connection at any point in your script. There's nothing preventing it nor there's a reason to discourage it. It's far more common than you'd think (for different reasons). However, considering the overhead that actually connecting to the database carries, I would advice you to not establish a connection on each iteration (this is the best practice, performance-wise)
What I would do:
First, connect to the source database, get the data you need
Not much to say here, you already did and it looks OK. Check, as you did, that there's at least one result before moving on. If the resultset is empty, don't go any further (exit)
If there was at least one row in the resultset, establish a connection to the destination database
Do this before starting to loop to avoid the performance penalty of establishing multiple connections. If you're looking at hundreds/thousands or more rows, you'll really notice the difference. Once the connection is established, move on
Loop through the source data and prepare the insert statements
You have a choice here. Either a) loop through the whole resultset and prepare one single insert with multiple rows which you'll insert all at once at the end of the loop or b) create a single insert (single row) on each iteration, run the insert and then move on to the next iteration.
There's reasons for and against both strategies. Choose the one that suits you better.
In algorythmical terms, your code is pretty much ready (just connect to the destination DB once before you start looping). You may benefit from using a parameterized query with bindings instead of writing the full insert string, but other than that you're pretty much there.

Answer 3

If you are running your own MySql server, you may do this in a simpler way using the Federated engine.
The federated table behaves like a normal table, but it stores the data in a remote database. With this engine you can do someting like this:
INSERT INTO remoteTable (values) SELECT * FROM localTable