Postgresql 我如何找到Postgres一个月的最大选项数?
我想了解每个用户在一个月内可选择的最高费率。以下是我的费率表:Postgresql 我如何找到Postgres一个月的最大选项数?,postgresql,Postgresql,我想了解每个用户在一个月内可选择的最高费率。以下是我的费率表: Member | Month | Rate Joe | Jan | 1 Joe | Jan | 2 Joe | Jan | 3 Joe | Feb | 1 Joe | Feb | 2 Joe | Feb | 2 Joe | Mar | 1 Joe
Member | Month | Rate
Joe | Jan | 1
Joe | Jan | 2
Joe | Jan | 3
Joe | Feb | 1
Joe | Feb | 2
Joe | Feb | 2
Joe | Mar | 1
Joe | Mar | 2
Joe | Mar | 2
Max | Jan | 1
Max | Jan | 1
Max | Jan | 1
Max | Feb | 2
Max | Feb | 2
Max | Feb | 2
Max | Mar | 3
Max | Mar | 3
Max | Mar | 3
Ben | Jan | 1
Ben | Jan | 2
Ben | Jan | 2
Ben | Feb | 1
Ben | Feb | 1
Ben | Feb | 1
Ben | Mar | 1
Ben | Mar | 1
Ben | Mar | 1
1月份,乔有利率选择权[1,2,3]。乔,在二月和三月,只有两个[1,2]。对于每个用户,我想显示一个月内可用的最大费率数(与所有月份相比)。结果表应如下所示:
Member | Max rates in one month
Joe | 3
Max | 1
Ben | 2
如何编写此查询?您可以通过一些聚合来完成此操作:
SELECT Member, MAX(rate_count) as "Max rates in one month"
FROM
(SELECT Member, Month, COUNT(DISTINCT RATE) rate_count FROM yourtable GROUP BY Member, Month) dt
GROUP BY Member;
首先,您需要按成员和月份分组并计算费率:
SELECT "Member", "Month", count(*) AS n FROM rates GROUP BY 1,2;
现在您有了每个成员每月的费率计数,您可以使用结果提取每个成员的最大值:
SELECT "Member", max(n) FROM (
SELECT "Member", "Month", count(*) AS n FROM rates GROUP BY 1,2) X
GROUP BY "Member";
还有其他方法,例如,使用窗口函数,但是想法总是一样的,我认为上面的代码清楚地说明了发生了什么