選擇從1數分列成2列

Question

我有了一個名爲type列其持有的兩個詞upvote或downvote我想通過a.question_id組再得2列，其中每一一個計數一個表。

select a.*, u.* from answers a 
left join users u using(user_id) 
left join vote_history v using(answer_id) 
where a.question_id = 4 and deleted < 4 
group by v.question_id 
order by votes desc 

例選擇

ColumnA | ColumnB | upvotes | downvotes 
---------+----------+---------+---------- 
record1A | record2B | 3  | 2 
record2A | record2B | 2  | 5 

是否有可能爲我做這件事而不做多個查詢，並沒有做一個子查詢？

Answer 1

這被稱爲一個樞軸。在MySQL中，可以使用具有CASE表達式的聚合函數將數據轉換爲列：

select a.*, u.*, 
    SUM(case when v.type='upvote' then 1 else 0 end) Upvotes, 
    SUM(case when v.type='downvote' then 1 else 0 end) Downvotes, 
from answers a 
left join users u using(user_id) 
left join vote_history v using(answer_id) 
where a.question_id = 4 and deleted < 4 
group by v.question_id 
order by votes desc 

Answer 2

這奧特工作：

select 
    a.*, 
    u.*, 
    SUM(IF(v.type = 'upvote',1,0) as upvotes, 
    SUM(IF(v.type = 'downvote',1,0) as downvotes, 
from answers a 
left join users u using(user_id) 
left join vote_history v using(answer_id) 
where a.question_id = 4 and deleted < 4 
group by v.question_id 
order by votes desc