Thanks Rick,
sorry in advance for the larger code snippet. The earlier one I had pared down to get to the heart of the question. This is the code I am actually using, and I used LEFT JOIN in the select that was trying to get Feedback_Occupation:
$set1=$wpdb->get_results ("
(SELECT pm4.meta_value as Feedback_Success,Feedback_Occupation,Feedback_Location,Feedback_Product,Feedback_Initials, object_id , rate, rtitle, excerpt from
(SELECT pm3.meta_value as Feedback_Occupation,Feedback_Location,Feedback_Product,Feedback_Initials, object_id , rate, rtitle, excerpt from
(SELECT pm2.meta_value as Feedback_Location,Feedback_Product,Feedback_Initials, object_id , rate, rtitle, excerpt from
(SELECT pm1.meta_value as Feedback_Product,Feedback_Initials, object_id , rate, rtitle, excerpt from
(SELECT pm.meta_value as Feedback_Initials, object_id , rate, rtitle, excerpt from
(SELECT object_id, meta.meta_value as rate, wposts.post_title as rtitle, wposts.post_excerpt as excerpt FROM $wpdb->term_relationships INNER JOIN $wpdb->posts wposts ON wposts.ID = object_id INNER JOIN $wpdb->postmeta meta ON meta.post_id = object_id WHERE term_taxonomy_id = '".$pcvalue."' AND wposts.post_type = 'reviews' AND wposts.post_status = 'publish' AND meta.meta_key = 'Feedback_Our_Rating' ORDER BY CAST(meta.meta_value as unsigned integer) DESC)
tb INNER JOIN wp_postmeta pm ON pm.post_id = tb.object_id WHERE pm.meta_key = 'Feedback_Initials')
tb1 INNER JOIN wp_postmeta pm1 ON pm1.post_id = tb1.object_id WHERE pm1.meta_key = 'Feedback_Product')
tb2 INNER JOIN wp_postmeta pm2 ON pm2.post_id = tb2.object_id WHERE pm2.meta_key = 'Feedback_Location')
tb3 LEFT JOIN wp_postmeta pm3 ON pm3.post_id = tb3.object_id WHERE (pm3.meta_key = 'Feedback_Occupation'))
tb4 INNER JOIN wp_postmeta pm4 ON pm4.post_id = tb4.object_id WHERE pm4.meta_key = 'Feedback_Success')
");
The above query will not return any feedback(results row) that does not have a row in postmeta for Feedback_Occupation. I only changed the second to the outermost INNER JOIN to a LEFT JOIN