Mysql: how to Calculating the difference between two avg?

Question

I have a table:

+------------+--------------+-------+------------+
| Name       | Nation       | Score | Game_date  |
+------------+--------------+-------+------------+
| Ginobili   | Argentina    |    48 | 2005-01-21 |
| Irving     | Australia    |    44 | 2014-04-06 |
| Kirilenko  | Soviet Union |    31 | 2006-11-11 |
| Kobe       | USA          |    81 | 2006-01-22 |
| LeBron     | USA          |    52 | 2014-12-06 |
| Mutombo    | Congo        |    39 | 1992-02-03 |
| Nowitzki   | Germany      |    48 | 2011-05-18 |
| PauGasol   | Spain        |    46 | 2015-01-11 |
| SteveNash  | Canada       |    42 | 2006-12-08 |
| TonyParker | France       |    55 | 2008-11-06 |
| YaoMing    | China        |    41 | 2009-02-23 |
| YiJianlina | China        |    31 | 2010-03-27 |
+------------+--------------+-------+------------+

I want to calculate the avg(score) of USA - avg(score) of China.

I have tried

select avg(score) from nba where nation = "USA" - 
avg(score) from nba where nation = "China";

But it is wrong!

Answer 1

You can use AVG and CASE WHEN :

SELECT AVG(CASE WHEN nation = 'USA' THEN score END) -
       AVG(CASE WHEN nation = 'China' THEN score END) AS result
FROM nba
WHERE nation IN ('USA', 'China');

Another option is to use UNION :

SELECT SUM(sub.r) AS result
FROM ( SELECT avg(score) AS r
       from nba 
       where nation = 'USA' 
       UNION ALL
       SELECT -avg(score) 
       from nba 
       where nation = 'China') AS sub