sql - sql从计数中找到平均值？

Question

所以我有一个查询：

select states, count(numberofcounty) 
from countytable 
group by states  
order by count(distinct numberofcounty) ASC;

它返回一个包含 2 列的表：从最少到最多的州数和县数。如何在单个实数中获得每个州有多少个县的平均值？

表结构为：

create table numberofcounty(
    numberofcounty text, 
    states text references states(states), 
);
create table states (
    states text primary key,  
    name text, 
    admitted_to_union text

);

Answer 1

您可以将当前查询用作获取每个州县数平均值的子查询

Select avg (c) average_counties
From (select count(numberofcounty) c
from countytable 
group by states) s

Answer 2

这可能是它？

countytable:
state|county
a     aa
a     ab
a     ac
b     ba


SELECT AVG(counties) FROM (SELECT state, COUNT(county) as counties FROM countytable GROUP BY state) as temp

结果：2

Answer 3

如果我没有遗漏任何东西，这基本上应该给您与其他答案中的双重分组方法相同的结果：

SELECT COUNT(numberofcounty) / COUNT(DISTINCT states)
FROM countytable
;