在 1 个查询中选择/求和具有不同条件的列 - select/sum of a column with different conditions in 1 query

select/sum of a column with different conditions in 1 query

本文关键字：条件求和查询选择 | 更新日期: 2023-09-27

$pos = select * from score_history where content_id = 6 && val = 1 
$neg = select * from score_history where content_id = 6 && val = -1

我想在一个查询中获取pos和neg分数但我不想使用加入

所以也许是某种 IF/case 陈述？

我有这个，但你可以猜到它失败

了

SELECT  count(*) as total , 
CASE 
    WHEN `val` =  1  THEN count(*) as `pos` 
    WHEN `val` = -1  THEN count(*) as `neg` 
END
FROM    score_history WHERE `content_id` = '46083' ";

有没有办法在不使用连接或子查询的情况下做到这一点？

您可以利用MySQL的灵活性来处理布尔值和整数：

SELECT  count(*) total, sum(val = 1) pos, sum(val = -1) neg
FROM    score_history 
WHERE   content_id = '46083';

只要条件为真，它就是 1。否则为 0。不需要案例，也不需要分组。

关闭！ CASE语句不会返回多个列，因此您需要 2 个CASE语句并将它们包装在 SUM() 中：

SELECT  count(*) as total 
  ,SUM(CASE WHEN `val` =  1  THEN 1 ELSE 0 END) as `pos`
  ,SUM(CASE WHEN `val` =  -1  THEN 1 ELSE 0 END) as `neg` 
FROM    score_history WHERE `content_id` = '46083' ;

SELECT
SUM(CASE WHEN `val` =  1  THEN 1 ELSE O END) AS pos_count, 
SUM(CASE WHEN `val` =  -1  THEN 1 ELSE O END) AS neg_count
FROM    score_history WHERE `content_id` = '46083';

试试这个。抱歉，我无法测试，这台笔记本电脑上没有数据库。

select 
count(*) as total,
sum(case val when  1 then 1 else 0 end) as pos,
sum(case val when -1 then 1 else 0 end) as neg
from score_history
where content_id = 6

不确定这是否是最佳答案(假设表中有很多行，您肯定希望在 val 列上有一个索引(，但这肯定应该有效 - 也假设您只有 1 和 -1 作为值：

SELECT count(*), val from score_history where content_id = 6 group by val;

你很接近;试试SUM函数：

SELECT  count(*) as total 
      , sum(CASE WHEN `val` =  1 THEN 1 ELSE 0 END) as `pos` 
      , sum(CASE WHEN `val` = -1 THEN 1 ELSE 0 END) as `neg` 
FROM    score_history 
WHERE  `content_id` = '46083';

select count(*) 
from score_history  
where content_id = 6 && 
      (val = -1 or val=1) 
group by val

我认为这个陈述应该有效，但我已经在 DBMS 上进行了测试。

 SELECT  count(*) as total ,
         count(case when val = 1 then 1 else null end) as pos,
         count(case when val = -1 then 1 else null end) as neg
 FROM    score_history 
 WHERE  `content_id` = '46083';

参见 SQLFIDDLE

好的，很多答案都很接近，但是每当您使用聚合函数时，都应该使用group by。

SELECT  count(*) as total 
      , (CASE WHEN `val` >= 0 THEN 'positive' ELSE 'negative' END) as interpreted_value
END
FROM    score_history 
WHERE  `content_id` = '46083'
GROUP BY (CASE WHEN `val` >= 0 THEN 'positive' ELSE 'negative' END);

如果您想在此处阅读如何使用group by和聚合函数：https://dev.mysql.com/doc/refman/5.0/en/group-by-functions.html