去掉最大年龄、最小年龄，不该用字段age_group去查

直接简单一条sql可以查出

select avg(age) from test_2001 a 

where not exists(select 1 from test_2001 having max(age)=a.age and min(age)=a.age)

SELECT * FROM test_2001 a

WHERE NOT EXISTS

(SELECT * FROM test_2001  WHERE age_group!=(select MAX(age_group) from test_2001) and a.id=id)

或者

SELECT * FROM test_2001 

WHERE NOT IN

(SELECT * FROM test_2001  WHERE age_group!=(select MAX(age_group) from test_2001))

exists要看sql返回的结果集是否与前面匹配，所以exsts括号的查询要跟前面有关联

你的sql中，exists括号里的查询跟前表没有任何关联，所以出不来结果

直接判断试试

select avg(age_group) from test_2001

where age_group<>(select max(age_group) from test_2001) and age<>(select min(age_group) from test_2001)

https://www.cnblogs.com/flzs/p/11542181.html

select avg(age_group) 

from test_2001 t

where not exists

(select 1

from 

(select 

    max(age_group) as max_aget,

    min(age_group) as min_age 

from test_2001

) t1

where t.age_group=t1.max_aget or t.age_group=t1.age_group

)