$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