sql - 需要使用复合主键计算不同的值

Question

我必须生成一份报告，显示不同地点的名称以及在这些地点提供独特类(class)的次数。报告只需要注明城市和编号。的独特类(class)，并按独特类(class)的最高数量排序。
我的问题是类(class)表有一个复合主键(类(class)开始日期和类(class)代码)

我运行这个 SQL 查询，它出现了一个错误，因为你不能计算多个列。但我不确定如何拆分计数。

SELECT 
    offlocation AS city, COUNT(distinct offbegindate,crscode) as no. of unique course offerings
FROM
    offering
GROUP BY offlocation
ORDER BY COUNT(distinct offbegindate,crscode) as no. of unique course offerings DESC;

任何帮助将不胜感激。

Answer 1

只是细微的调整，计数不同的“行类型”:

SELECT 
    offlocation AS city,
    COUNT(distinct (offbegindate,crscode)) as no_of_unique_course_offerings
FROM
    offering
GROUP BY offlocation
ORDER BY no_of_unique_course_offerings DESC;