How Can I Update an SQL database with a foreach loop? - php

Question

I have a photo gallery and want to update multiple captions at once via form input. I've tried to research this but I think I'm in way over my head. This is what I have so far but it's not working..
The data is saved in an SQL table called "gallery". An example row might look like:
gallery_id(key) = some number
product_id = 500
photo = photo.jpg
caption = 'look at this picture'
My form inputs are generated like this:
$sql = mysql_query("SELECT * FROM gallery WHERE product_id = 500");
while($row = mysql_fetch_array($sql)) {
$photo=$row['photo'];
$caption=$row['caption'];
echo '<img src="$photo"/>';
echo '<input name="cap['.$caption.']" id="cap['.$caption.']" value="'.$caption.'" />';
}
So once I submit the form I start to access my inputs like this but I hit a wall..
if( isset($_POST['cap']) && is_array($_POST['cap']) ) {
foreach($_POST['cap'] as $cap) {
mysql_query("UPDATE gallery
SET caption=$caption
WHERE ???????");
}
}
I don't know how to tell the database where to put these inputs and as far as I can tell you can't pass more than one variable in a foreach loop.

Answer 1

$_POST is an array, if im not wrong (I can't test it now and im new in PHP) you can do
foreach ($_POST as $p) {
$id=$p['id'];'
$cap=$p['caption'];
mysql_query("UPDATE gallery
SET caption=$cap
WHERE photoid=$id");
}

Answer 2

A few things to consider
Updating many rows is tricky. I see it's already answered so I'm not trying to give a better answer, but some non-trivials for those who find this thread by searching for update sql many rows or similar. It's a common scenario to update several rows, and I've seen many codes with the "one update a time" approach (described above) which can be quite slow. In many cases there are better ways. No one-fits-all solution here, so I tried to split up the possible techniques by what you have and where you're going:
Multiple rows, same dataIf you want to set the same caption for many rows, UPDATE ... WHERE ID IN (...) - it's an edge case but it gives you a noticeable performance boost
Multiple rows, different data, at onceWhen updating several rows, consider using a CASE structure, as explained in this article:
sql update multiple rows with case in php
TransactionsIf you're using InnoDB tables (which is quite likely), you may want to do the whole thing in one transaction. It's a lot faster.
Index updatesIf you're updating many-many rows (like thousands), it can make sense to disable keys before and enable them afterwards. This way SQL can save a lot of index updates while it happens, and only do it for the final result.
Delete + reinsertIf you're updating many fields per record, and there are no triggers and other magic consequences of deleting your rows, it can be faster to delete them WHERE ID IN(...) and then do a multi-insert. Don't do this if you're only updating fixed-size integers like counters.
Then again:
If your table has many concurrent reads & writes, and you're updating only a few (up to a hundred) rows, stick with the one-by-one approach. Especially if it's MyISAM.
Try before you buy; these techniques all depend on data itself too. But it's worth to go some extra miles to find the best.