如何在額外列中的一個sql查詢中結合2個組的查詢結果？

Question

考慮以下mdl_course_completions表描述了用戶結業：

id,bigint 
userid,bigint 
course,bigint 
timeenrolled,bigint 
timestarted,bigint 
timecompleted,bigint 
reaggregate,bigint 

要確定的，如果一門課程已經由學生完成，我用的timecompleted領域的謂語。

當此字段爲空時，學生尚未完成課程，但是當此字段不爲空時，表示學生已完成課程。

因此，成品按課程課程的學生人數的計數爲：

SELECT mdl_course.fullname,count(*) as "number of students that didn't finish courses" 
FROM mdl_course_completions 
    INNER JOIN mdl_course on mdl_course.id = mdl_course_completions.course 
WHERE timecompleted IS NOT NULL 
GROUP BY mdl_course.fullname 
; 

結果是：

| course name | number of students that finish courses | 
|-------------|----------------------------------------| 
| course 1 | 50          | 
| course 2 | 200         | 
| course 3 | 120         | 

和學生人數的計數沒'T完成當然課程是由：

SELECT mdl_course.fullname,count(*) as "number student that didn't finish courses" 
FROM mdl_course_completions 
    INNER JOIN mdl_course on mdl_course.id = mdl_course_completions.course 
WHERE timecompleted IS NULL 
GROUP BY mdl_course.fullname 
; 

結果是：

| course name | number of students that didn't finish courses | 
|-------------|-----------------------------------------------| 
| course 1 | 12           | 
| course 2 | 12           | 
| course 3 | 120           | 

我不知道如何可以結合這2個查詢在一個查詢得到的結果在一個額外的列如：

| course name | number of students that finish courses | number of students that didn't finish courses | 
|-------------|------------------------------------|-------------------------------------------| 
| course 1 | 50         | 12          | 
| course 2 | 200        | 12          | 
| course 3 | 120        | 120          | 

我使用postgresql.In我看來，這種東西與數據庫系統無關。我只是不知道如何繼續將這兩個查詢合併成一個額外的列與GROUP BY子句。

Answer 1

使用條件聚合。

SELECT mdl_course.fullname 
,SUM((timecompleted IS NOT NULL)::int) as "number student that finish courses" 
,SUM((timecompleted IS NULL)::int) as "number student that didn't finish courses" 
FROM mdl_course_completions 
INNER JOIN mdl_course on mdl_course.id = mdl_course_completions.course 
GROUP BY mdl_course.fullname 

Answer 2

和PostgreSQL 9.4，你可以使用FILTER子句與聚合函數：

count(*) FILTER (WHERE timecompleted IS NOT NULL)