選擇行的總數

Question

的順序我有持有AA用戶ID，他們投票支持這樣的用戶表：

uid | voted_for 
1 | 3 
2 | 3 
3 | 1 

是我的目標做的是基於有多少人爲了UID已經投票支持該uid。但我不知道該怎麼做。

所以，最終的結果將是：

uid | Total_Votes 
3 |  2 
1 |  1 
2 |  0 

希望你能幫助解釋來構造SQL這樣做的最佳方式。

Answer 1

這個簡單的查詢將產生你所要求的輸出：

select voted_for as uid, count(*) as total_votes 
from users 
group by 1 
order by 2 desc 

---

如果您想了解在輸出的每個用戶的所有數據，用戶加入到本身：

select u.*, count(v.uid) as total_votes 
from users u 
left join users v on v.voted_for = u.uid 
group by 1,2,3,4,5 -- put as many numbers here as there are columns in the users table 
order by total_votes desc 

這第二個查詢會給total_votes零分，如果沒有人投給了用戶。

或者，您也可以只選擇那些列你想：

select u.uid, u.name, count(v.uid) as total_votes 
from users u 
left join users v on v.voted_for = u.uid 
group by 1,2 
order by 3 desc 

```

只返回贏家，這樣做：

select u.uid, u.name, count(*) as total_votes 
from users u 
left join users v on v.voted_for = u.uid 
group by 1,2 
having count(*) = (
    select max(c) from (
     select count(*) as c from users group by voted_for)) 
order by 3 desc 

Answer 2

也許像這樣將有助於自身加入表：

SELECT u.*, voted_for_cnt 
FROM users u 
    LEFT JOIN (
     SELECT voted_for, count(1) voted_for_cnt 
     FROM users 
     GROUP BY voted_for 
    ) t ON u.uid = t.voted_for 
ORDER BY t.voted_for_cnt DESC 

• SQL Fiddle Demo