如何从SQL中提取唯一值？

Question

例如，假设我有以下表

user_id, score 
1,5 
1,4 
2,9 
3,7 
3,6 
3,15 
4,8 
4,11 

我想返回USER_ID 2的查询，因为它是不重复的唯一USER_ID

Answer 1

SELECT user_id FROM table GROUP BY(user_id) HAVING COUNT(user_id) = 1 

Answer 2

select user_id 
from tableName 
group by user_id 
having count(*) = 1; 

Answer 3

SELECT user_id 
FROM table 
GROUP BY user_id 
HAVING (COUNT(user_id)=1)