选择从 1 列计数到 2 列答案

【问题标题】：Select Count From 1 Column Into 2 Columns选择从 1 列计数到 2 列
【发布时间】：2013-01-01 11:51:16
【问题描述】：

我有一个表，有一个名为 type 的列，它包含两个词之一 upvote 或 downvote 我想按 a.question_id 分组，然后得到 2 列，每列计数。

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

示例选择

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

我是否可以在不执行多个查询且不执行子查询的情况下执行此操作？

【问题讨论】：

标签： mysql select join count group-by

【解决方案1】：

这被称为枢轴。在 MySQL 中，您可以使用带有 CASE 表达式的聚合函数将数据转换为列：

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

【讨论】：

【解决方案2】：

这一切都有效：

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

【讨论】：