如何嵌套兩個count（）語句？

Question

想象一下三列表，比如說TRAVELS，其中列表NAME和COUNTRY他們曾經去過，而DATE，這樣就可以有重複的NAME + COUNTRY行。

- >我不想顯示實際NAME值，也沒有實際COUNTRY值 - 只是他們計數，只爲那些已探訪超過1個國家。

這將是像這樣的列表：

| 9 | 2 | 意思9人已訪問2個國家，
| 4 | 3 | 意思4人已訪問3個國家，
| 2 | 4 | 意味着2人訪問了4個國家，
等

我在SQL很糟糕，但我想我需要建立某種嵌套查詢的：

select count(t1.NAME) 
from TRAVELS t1 
where (
    select count(distinct COUNTRY) 
    from TRAVELS t2 
    where t1.name = t2.name 
    and count(t2.COUNTRY) > 1 
) 

顯然這不起作用，但我希望它顯示我的一般意圖。我一直無法找到任何關於這種嵌套雙數的東西，儘管一旦我看到它可能很簡單。

Answer 1

select country_count, count(*) as people_count 
    from (select name, count(distinct country) as country_count 
      from travels 
     group by name 
     having count(distinct country) > 1) 
group by country_count 

當然這假定名稱是唯一的。如果不是，你可能想用user_id代替名字。

Answer 2

這裏試試：http://sqlfiddle.com/#!2/b0762/3

查詢：

select 
    count(*) as people, 
    countries_visited 
from (
    select 
     name, 
     count(distinct country) as countries_visited 
    from travels 
    group by name 
    having count(distinct country) > 1 
) s 
group by countries_visited 
order by people desc, countries_visited desc