Shortcode to output unique donor count for specific form (needs caching)

give-count-unique-donors.php

<?php
/**
 * Give count unique donors
 * 
 * @param array $atts [
 *  'form_id' => int The form ID to count donors for
 * ]
 */
function kia_give_donor_count( $atts ) {

    global $wpdb;

    $atts = shortcode_atts( array(
        'form_id' => 0
    ), $atts, 'give_donor_count' );

    if ( empty ( $atts[ 'form_id' ] ) ) {
        return;
    }

    $sql = $wpdb->prepare( "
        SELECT 
            COUNT( DISTINCT (rel2.meta_value ) ) as donor_count
        FROM {$wpdb->posts} AS posts
            LEFT JOIN {$wpdb->prefix}give_donationmeta AS rel ON 
                posts.ID = rel.donation_id
            LEFT JOIN {$wpdb->prefix}give_donationmeta AS rel2 ON
                posts.ID = rel2.donation_id
        WHERE
            posts.post_type = 'give_payment' AND
            posts.post_status = 'publish' AND 
            rel.meta_key = '_give_payment_form_id' AND
            rel.meta_value = %d AND
            rel2.meta_key = '_give_payment_donor_email'
        ",
        intval( $atts[ 'form_id' ] )
    );

    $result = $wpdb->get_var( $sql );

    return '<p class="donor_count">' . sprintf( esc_html__( 'Total donors: %d', 'your-textdomain' ), $result ) . '</p>';
}
add_shortcode( 'give_donor_count', 'kia_give_donor_count' );