多个表上的SQL AVG函数

Question

I have two more or less identical tables:

test_score

id  int(11) PK AI
user_id int(11) 
module_id   int(11) 
score   double 
number_correct  int(11) 
correct_total   int(11) 
timestamp   datetime 
phase_id    int(11) 
medal_id    int(11) 
team_id int(11) 
status_id   int(11) 

And

id  int(11) PK AI
user_id int(11) 
module_id   int(11) 
score   double 
number_correct  int(11) 
correct_total   int(11) 
timestamp   datetime 
phase_id    int(11) 
medal_id    int(11) 
team_id int(11) 
status_id   int(11) 

Now the reason for these two being so identical is that they contain data from each of their own component in my system.

Now i want to use the AVG() function to find the average score of these two tables combined.

is this possible?

Answer 1

SELECT user_id, average_score = AVG(score)
FROM (
    SELECT user_id, score FROM test_score1
    UNION ALL
    SELECT user_id, score FROM test_score2
) AS subquery
GROUP BY user_id

Answer 2

Stop thinking in terms of tables, but rather "result sets".

Break the problem into two components..

First, how do I create a "result set" of the data I want from these two tables? Simplest solution, the union statement

select * from <table1>
union
select * from <table2>

Now, take the above result set and compute the average

select xx.module_id,avg(xx.score) as AvgModuleScore
from
(
    select * from <table1>
    union
    select * from <table2>
) xx