$wpdb not seeming to work with - custom-post-type

Question

I'm using the following code to populate a WordPress dropdown menu with all the unique values from a custom field:
<form name="search" action="" method="get">
<select name="stateprov">
<option>Select...</option>
<?php
$metakey = 'state_prov';
statesProvs = $wpdb->get_col($wpdb->prepare("SELECT DISTINCT meta_value FROM $wpdb->postmeta WHERE meta_key = %s ORDER BY meta_value ASC", $metakey) );
if ($statesProvs) {
foreach ($statesProvs as $stateprov) {
echo "<option value=\"" . $stateprov . "\">" . $stateprov . "</option>";
}
}
?>
</select>
<input type="submit" value="search" />
</form>
However, it takes nothing from the DB so the popup list is empty.
Trying a different query like
$statesProvs = $wpdb->get_col( "SELECT ID FROM $wpdb->posts WHERE post_author = 2" );
Works as expected. I get a popup with a bunch of Post ID's in it. But the query that's supposed to work on my custom metadata just brings up an empty menu (and print_r reveals an empty array).
The data is definitely in the DB... what am I doing wrong?
It may also be significant that I'm using a custom metabox PHP class that writes all the custom-created field keys and values into the value of the _custom_meta metakey. If I've put that correctly. Thus:
a:61:{s:10:"state_prov";s:2:"CA";s:13:"vertical_drop";s:13:"3100ft / 945m";s:14:"base_elevation";s:14:"7953ft / 2424m";s:16:"summit_elevation";s:15:"11053ft / 3369m";s:12:"skiable_area";s:10:"3500 acres";s:16:"average_snowfall";s:14:"400in / 1016cm";s:13:
Etc. Is this way of storing the custom metadata preventing wpdb from accessing it properly?
Thanks!

Answer 1

you must use global $wpdb before you start the wpquery, refer codex for more details
<form name="search" action="" method="get">
<select name="stateprov">
<option>Select...</option>
<?php
global $wpdb;
$metakey = 'state_prov';
$wpdb->get_col($wpdb->prepare("SELECT DISTINCT meta_value FROM $wpdb->postmeta WHERE meta_key = %s ORDER BY meta_value ASC", $metakey) );
if ($statesProvs) {
foreach ($statesProvs as $stateprov) {
echo "<option value=\"" . $stateprov . "\">" . $stateprov . "</option>";
}
}
?>
</select>
<input type="submit" value="search" />
</form>

Answer 2

Etc. Is this way of storing the custom metadata preventing wpdb from
accessing it properly?
Yes.
In general, Wordpress isn't capable of searching or sorting through serialized PHP arrays.
The most viable solution here is to store this data point ('state_prov') in its own custom field. This will allow you to naturally search and sort using $wpdb. It also seems like you want this custom field to be private/hidden, in which case you'll want to prefix it with an underscore: '_state_prov'.
Another option, assuming the above isn't viable for your needs, is to use MySQL String Functions; but this is far from ideal or optimized (especially if the database grows significantly.) Rough, untested example below.
// Assuming all state_prov are exactly 2 characters in length
$identifier = 's:10:"state_prov";s:2:"';
$identifier_length = strlen($identifier);
$wpdb->get_col($wpdb->prepare("SELECT DISTINCT(SUBSTR(`meta_value`, INSTR(`meta_value`, '$identifier')+$identifier_length, 2)) as `state_prov` FROM $wpdb->postmeta WHERE `meta_key`=%s ORDER BY `state_prov` ASC", $metakey) );