Populate Checkboxes on Edit Form - php

Question

I have an edit page that gets populated when accessed. The input values work fine, but I'm having a hard time ticking the category checkboxes. I get the information from two tables. One that displays all the categories and the other one that gets the categories associated with the item.
The following code doesn't work because the second while statement finishes its loop in the first round. Is there an appropriate way to do this?
<?php $check_cats = mysql_query("SELECT * FROM item_categories WHERE itemid = '$itemid'") or die(mysql_error()); ?>
<?php $result = mysql_query("SELECT * FROM categories ORDER BY cname") or die(mysql_error()); ?>
<?php while($row = mysql_fetch_array( $result )) { ?>
<input type="checkbox" id="<?php echo $row['cname']; ?>" name="cat[]" value="<?php echo $row['id']; ?>"
<?php while($check_cat_rows = mysql_fetch_array( $check_cats )) {
if ($check_cat_rows['catid'] == $row['id']) {
echo 'checked="yes"';
}
}
} ?>
My Two tables:
TABLE `item_categories`
`id`
`itemid`
`catid`
TABLE `categories`
`id`
`cname`

Answer 1

Your basic structure could use improvement. Rather than two separate queries, and two nested loops, you could be using a single query which JOINS to the two tables together. Part of the joined data would be the "checked" flag, which you can check for within the loop and output the appropriate html.
SELECT ..., categories.id AS checked
FROM item_categories
LEFT JOIN categories
ON (item_categories.catid = categories.id)
and then while looping:
while($row = mysql_fetch_assoc()) {
$flag = ($row['checked') ? ' checked="yes"' : ''
...
}

Answer 2

Your whole thing is structured incorrectly, you can't assume the two results will line up perfectly, and your loops are wrong. Try this:
SELECT *,
(case when id IN
(SELECT catid FROM item_categories WHERE itemid = '$itemid')
then 1 else 0 end) checked
FROM categories ORDER BY cname
Now you just run the one query and have a nice little $row['checked'] to use!
SELECT *,
(case when categories.id IS NOT NULL
then 1 else 0 end) checked
FROM item_categories
LEFT JOIN categories
ON (item_categories.catid = categories.id)
WHERE itemid = '$itemid'
Improved based on hybrid between marc B and mine... Only efficiency difference is that the query handles testing the validity of categories.id instead of the php

Answer 3

I don't really understand your question, but you want the check box to be checked when the page loads? In that case you have to add "checked" to the tag
<input type="checkbox" name="option2" value="Butter" checked>

Answer 4

Something like this?
<?php
$query = <<<query
SELECT
c.id,
c.cname,
ifnull(ic.catid, '', 'checked="yes"') as checked
FROM
categories c
LEFT JOIN item_categories ic
ON ic.itemid = '$itemid'
AND ic.catid = c.id
ORDER BY
c.cname
query;
$result = mysql_query($query) or die(mysql_error());
while ($row = mysql_fetch_array($result)) { ?>
<input type="checkbox" id="<?=$row['cname'];?>" name="cat[]" value="<?=$row['id'];?>" <?=$row['checked'];?>>
<?
}
?>