如何计算涉及多个表的多个列的平均值和计数？

Here are my different tables:

这是我不同的表格:

computers (id,name)
monitors (id,name)
computer_monitor (id, computer_id,monitor_id)
useractivity (id,userid,timestamp,computer_monitor_id,ip)
useropinion (id,userid,computer_monitor_id,timestamp,rating)
user (id,name,email)
com