MySQL的SELECT COUNT DISTINCT

Question

我從下面的查詢

SELECT co.mobility_theme, COUNT(DISTINCT ca.user_id) AS n 
FROM courses_apply AS ca LEFT JOIN dy41s_courses AS co ON ca.course_id = co.id 
WHERE ca.submission IS NULL AND ca.`call` LIKE '1b' 
GROUP BY co.mobility_theme 
ORDER BY co.mobility_theme ASC 

得到如下結果總（133）

mobility_theme n 

1 =>70 

4 =>18 

5 =>45 

我想要做的是：爲每個移動主題（1,2和5），我有重複的用戶，所以總唯一的user_id應該是（130），我無法通過（移動性主題）獲得唯一的user_id組，任何幫助？

Answer 1

一種方法是任意選擇一種移動性主題。這種方法選擇最小值的那個：

select mobility_theme, COUNT(*) as Numusers 
from (select ca.user_id, MIN(co.mobility_theme) as mobility_theme 
     from courses_apply ca LEFT JOIN 
      dy41s_courses co 
      ON ca.course_id = co.id 
     WHERE ca.submission IS NULL AND ca.`call` LIKE '1b' 
     group by ca.user_id 
    ) caco 
group by mobility_theme