Question

我已经知道如何计算我的数据库中有多少天？

SELECT 
       COUNT(DISTINCT DATE (TIME)) AS distinct_days
FROM table;

但是当我尝试计算不同的数周或数月时，我发现的唯一解决方案是超慢......

几个月：

 SELECT 
       COUNT(DISTINCT CONCAT (EXTRACT(YEAR FROM TIME),EXTRACT(MONTH FROM TIME))) AS distinct_months
FROM table;

几周

SELECT 
       COUNT(DISTINCT CONCAT (EXTRACT(YEAR FROM TIME),EXTRACT(MONTH FROM TIME), EXTRACT(WEEK FROM TIME))) AS distinct_weeks
FROM table;

您有什么想法进行优化吗？

（更新）注意：

COUNT(DISTINCT DATE_TRUNC('week', time)) AS distinct_weeks

和

 COUNT(DISTINCT CONCAT (EXTRACT(YEAR FROM TIME),EXTRACT(MONTH FROM TIME), EXTRACT(WEEK FROM TIME))) AS distinct_weeks

没有相同的结果（我想要第二个）！

使用COUNT(DISTINCT DATE_TRUNC('week', time))，您有53种可能性，COUNT(DISTINCT CONCAT (EXTRACT(YEAR FROM TIME),EXTRACT(MONTH FROM TIME), EXTRACT(WEEK FROM TIME)))可能是无限（例如2014-01第1周与2013-01第1周不同）......

Answer 1

最后，我发现了至少两倍的东西：

表示不同的周：

SELECT COUNT(distinct_weeks)
FROM (SELECT CONCAT(EXTRACT(YEAR FROM TIME),EXTRACT(MONTH FROM TIME),EXTRACT(WEEK FROM TIME)) AS distinct_weeks
      FROM table
      GROUP BY EXTRACT(YEAR FROM TIME),
               EXTRACT(MONTH FROM TIME),
               EXTRACT(WEEK FROM TIME)) t

不同月份：

SELECT COUNT(distinct_months)
FROM (SELECT CONCAT(EXTRACT(YEAR FROM TIME),EXTRACT(MONTH FROM TIME)) AS distinct_months
      FROM table
      GROUP BY EXTRACT(YEAR FROM TIME),
               EXTRACT(MONTH FROM TIME)) t

Answer 2

也许简单地截断日期会更快，因为您不需要字符串转换和连接：

SELECT 
  COUNT(DISTINCT DATE_TRUNC('week', mytime)) AS distinct_weeks,
  COUNT(DISTINCT DATE_TRUNC('month', mytime)) AS distinct_months,
  COUNT(DISTINCT DATE_TRUNC('year', mytime)) AS distinct_years
FROM mytable;

在Postgresql中统计不同的日历周，月

2 个答案: