在1个查询中选择/总和具有不同条件的列

Question

$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得pos和neg分数，但我不想使用join

那么也许是某种IF /案例陈述？

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执行此操作？

Answer 1

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CASE，也不需要GROUP BY。

Answer 2

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

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' ;

Answer 3

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';

Answer 4

尝试这个。 对不起，我无法测试，这台笔记本电脑没有数据库。

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

Answer 5

不确定这是否是最好的答案（并且你肯定希望在你的val列上有一个索引，假设表中有很多行）但这当然可以工作 - 还假设你只有1和-1作为值：

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

Answer 6

你很亲密; 尝试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';

Answer 7

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。

Answer 8

 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';

请参见SQLFIDDLE

Answer 9

好的，很多这些答案都很接近，但无论何时使用聚合函数，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);

如果您想了解如何在此处使用group by和aggregate函数： https ： //dev.mysql.com/doc/refman/5.0/en/group-by-functions.html