sqlite3: COUNT & EXCEPT not working as expected - sql

Question

I'm fairly new to SQL but having searched the internet for an answer to this I still cannot get my COUNT and EXCEPT statements to select what I want.
My Database:
sqlite> CREATE TABLE Football(Team TEXT, Player TEXT, Age INTEGER, primary key(Team, Player));
sqlite> .separator ,
sqlite> .import databaseTest Football
sqlite> .headers on
sqlite> .mode col
sqlite> SELECT Team, Player, Age FROM Football ORDER BY Team;
Team Player Age
---------- ---------- ----------
Arsenal Cech 38
Arsenal Giroud 29
Arsenal Sanchez 28
Arsenal Walcott 27
Chelsea Costa 29
Chelsea Courtois 25
Chelsea Hazard 26
Chelsea Willian 26
Liverpool Can 23
Liverpool Coutinho 24
Liverpool Wjinaldum 25
Liverpool Woodburn 17
Manchester Aguero 29
Manchester Jesus 19
Manchester Silva 28
Manchester Toure 34
Manchester De Gea 26
Manchester Felliani 29
Manchester Rooney 32
Manchester Schweinste 35
Tottenham Delle Ali 22
Tottenham Kane 24
Tottenham Rose 24
Tottenham Vertonghen 27
What I want to do is SELECT the COUNT of teams that do not have a player over the age of 30. So the select statement should be 3 (Chelsea, Liverpool, Tottenham).
This is the statement I've tried and assumed would work:
sqlite> SELECT COUNT(DISTINCT Team) FROM Football
...> EXCEPT
...> SELECT COUNT(DISTINCT Team) FROM Football WHERE Age > 30;
COUNT(DISTINCT Team)
--------------------
6
But as you can see it returns '6'. What am I doing wrong and how can I get the correct result?

Answer 1

Here is another way. Look at the maximum age for each team:
SELECT COUNT(*)
FROM (SELECT Team
FROM Football
GROUP BY Team
HAVING MAX(Age) <= 30
) t;
You can also use EXCEPT, but this also requires a subquery. You need to do the set operation before doing the count:
SELECT COUNT(DISTINCT TEAM)
FROM (SELECT Team FROM Football
EXCEPT
SELECT Team FROM Football WHERE Age > 30
) t;
Strictly speaking, this query could use COUNT(*) rather than COUNT(DISTINCT). However, it can be troublesome to remember that EXCEPT (like UNION) removes duplicate values.