Use a PHP variable within an SQL statement

I am using wordpress and I have an SQL statement (as below) to gather the tags for a specific category. The only issue with this is that the table names may change (i.e. they will not always be wp_*). The function $pfx = $wpdb->base_prefix; returns what the wordpress database prefix is.

How can I use $pfx inside my SQL statement below (instead of wp_ ).

global $wpdb;
$tags = $wpdb->get_results
SELECT DISTINCT terms2.term_id as tag_id, as tag_name, null as tag_link, t2.count as post_total
        wp_posts as p1
        LEFT JOIN wp_term_relationships as r1 ON p1.ID = r1.object_ID
        LEFT JOIN wp_term_taxonomy as t1 ON r1.term_taxonomy_id = t1.term_taxonomy_id
        LEFT JOIN wp_terms as terms1 ON t1.term_id = terms1.term_id,

        wp_posts as p2
        LEFT JOIN wp_term_relationships as r2 ON p2.ID = r2.object_ID
        LEFT JOIN wp_term_taxonomy as t2 ON r2.term_taxonomy_id = t2.term_taxonomy_id
        LEFT JOIN wp_terms as terms2 ON t2.term_id = terms2.term_id

        t1.taxonomy = 'category' AND p1.post_status = 'publish' AND terms1.term_id IN (".$args['categories'].") AND
        t2.taxonomy = 'post_tag' AND p2.post_status = 'publish'
        AND p1.ID = p2.ID
    ORDER BY post_total DESC



Change for example

LEFT JOIN wp_term_relationships 


LEFT JOIN ".$pfx."term_relationships 

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