Average and max
Code:
mysql> select avg(cp_pr_history.google_pr) from cp_domains,cp_pr_history where client_id=1 and cp_domains.domain_id=cp_pr_history.domain_id group by cp_domains.client_id;
+------------------------------+
| avg(cp_pr_history.google_pr) |
+------------------------------+
| 2.8602 |
+------------------------------+
mysql> select max(cp_pr_history.google_pr) from cp_domains,cp_pr_history where client_id=1 and cp_domains.domain_id=cp_pr_history.domain_id group by cp_domains.client_id;
+------------------------------+
| max(cp_pr_history.google_pr) |
+------------------------------+
| 7 |
+------------------------------+