How to rename a custom field?

Question

I have 3000 posts with that custom field name 'refer' and i need to change it to 'ref' for theme needs, instead of doing it manually i need a query or technique to change them all.

Question 1
I am trying to rename a custom field, from 'refer' to ref. what is the query needed to rename a custom filed. it would be better if its done using function.php(instead of doing in phpmyadmin).

Question 2
I also have a lot of unwanted custom fields which are need to be deleted, how should i go about. help me out guys.

Answer 1

I found the answer

UPDATE `wp_postmeta` SET `meta_key` = 'ref' WHERE `meta_key` = 'refer'

use this part in your SQL

Reference

Answer 2

If you're trying to delete a post's custom field, then please edit your post, select 'Custom Fields' from the screen options, and scroll to the bottom, below the post editor. You'll find all your custom fields listed out there, you can edit their name, and change their values too. But ONLY those which you've added while adding / editing the post. Not the ones that WP creates for each post. DO NOT DELETE THE CUSTOM FIELDS WHICH YOU HAVEN'T ADDED!

Answer 3

An cleaner version of @t31os's answer :

global $wpdb;
$querystr = $wpdb->prepare( "UPDATE $wpdb->postmeta SET meta_key = '%s' WHERE meta_key = '%s'",'new','old');
$result = $wpdb->get_results ( $querystr );