从一个选择中除以两个计数

Question

我有一张这样的桌子：

date(timestamp) Error(integer)   someOtherColumns

我有一个查询要选择特定日期的所有行：

SELECT * from table
WHERE date::date = '2010-01-17'  

现在我需要计算 Error 等于 0 的所有行（从那天开始），然后除以所有行的计数（从那天开始）。

所以结果应该是这样的

Date(timestamp)      Percentage of failure
2010-01-17           0.30

数据库很大，有几百万行..

如果有人知道如何做到这一点，那就太好了——从一天到另一天。

Date(timestamp)      Percentage of failure
2010-01-17           0.30
2010-01-18           0.71
and so on

Answer 1

这个呢（如果error 可能只有 1 和 0）：

select
   date,
   sum(Error)::numeric / count(Error) as "Percentage of failure"
from Table1
group by date

或者，如果error 可以是任何整数：

select
   date,
   sum(case when Error > 0 then 1 end)::numeric / count(Error) as "Percentage of failure"
from Table1
group by date

---

刚刚发现我已经数过not 0（假设错误是在Error != 0 时），并且没有将空值计入帐户（不知道您想如何处理它）。所以这是另一个查询，它将空值视为 0，并以两种相反的方式计算失败百分比：

select
    date,
    round(count(nullif(Error, 0)) / count(*) ::numeric , 2) as "Percentage of failure",
    1- round(count(nullif(Error, 0)) / count(*) ::numeric , 2) as "Percentage of failure2"
from Table1
group by date
order by date;

sql fiddle demo

Answer 2

试试这个

select cast(data1.count1 as float)/ cast(data2.count2 as float) 
 from (
select count(*) as count1 from table date::date = '2010-01-17' and Error = 0) data1, 

(select count(*) as count1 from table date::date = '2010-01-17') data2

Answer 3

SELECT date
     , round(count((error = 0) OR NULL) / count(*)::numeric, 2) AS percent_fail
FROM   tbl
GROUP  BY 1
ORDER  BY 1;

如果error 可以是NULL，这甚至可以工作。

-> SQLfiddle demo.

在这个密切相关的问题下还有更多（包括对性能的影响）：
Compute percents from SUM() in the same SELECT sql query

计数方法的比较和基准this related answer on dba.SE。

Answer 4

您可以使用 generate_series 并从那里获取它。

像这样：

WITH CTE AS 
(
     SELECT 
         m
        --,extract('year'  FROM m) AS theyear
        --,extract('month' FROM m) AS themonth
        --,extract('day' FROM m) AS theday

        ,(SELECT COUNT(*) AS cnt FROM  table WHERE date::date = m AND Error = 1) AS data1 
        ,(SELECT COUNT(*) AS cnt FROM  table WHERE date::date = m) AS data2 
    FROM  
    (
        SELECT generate_series('2012-04-01'::date, '2016-01-01'::date, interval '1 day') AS m
    ) AS g 
) -- END OF CTE 

SELECT 
      m
     ,COALESCE(data1 * 100.0 / NULLIF(data2, 0.0), 0.0) AS ErrorPercentage
FROM CTE

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