我有一个查询，该查询可以正确计算平台中每个课程的学生人数，但是在学生人数= 0的情况下，它不会返回结果。如何修改以下查询以包括此内容？
SQL：
/* MSSQL
Name: Courses using the TDC themes with student enrolment count
Description: Returns a list of courses using a TDC theme and the number of enrolled students in each course.
*/
select
concat('<a target="_blank" href="%%wwwroot%%/course/index.php?categoryid=',cat.id,'">', cat.name,'</a>') as "Category",
concat('<a target="_new" href="%%wwwroot%%/course/view.php?id=',course.id,'">',course.fullname,'</a>') as "Course",
course.theme as "Theme",
count(course.id) as "Students"
from prefix_course as course
join prefix_context as context on context.instanceid = course.id
join prefix_role_assignments as ra on ra.contextid = context.id
join prefix_course_categories as cat on cat.id = course.category
where context.contextlevel = 50 and ra.roleid = 5 and course.theme like 'tdc%'
group by cat.id, cat.name, course.id, course.fullname, course.theme
order by cat.name
实际结果：
| Category | Course | # of Students |
|-----------|--------------------|---------------|
| Baking | Baking 101 | 7 |
| IT | Intro to Excel | 9 |
预期结果：
| Category | Course | # of Students |
|-----------|--------------------|---------------|
| Baking | Baking 101 | 7 |
| IT | Web Programming | 0 |
| IT | Intro to Excel | 9 |
| Chemistry | Chemical Reactions | 0 |
我已经搜索了一些关于SO的问题（例如here），但无法将答案转化为我的情况。
答案 0 :(得分：0)
如果将prefix_course表移至左外部联接并使用sum（if（course.id为null，0,1））而不是sum，则它应该起作用：
select
concat('<a target="_blank" href="%%wwwroot%%/course/index.php?categoryid=',cat.id,'">', cat.name,'</a>') as "Category",
concat('<a target="_new" href="%%wwwroot%%/course/view.php?id=',course.id,'">',course.fullname,'</a>') as "Course",
course.theme as "Theme",
sum(iif(course.id is null,0,1)) as "Students"
from prefix_context as context
inner join prefix_role_assignments as ra on ra.contextid = context.id
inner join prefix_course_categories as cat on cat.id = course.category
left outer join prefix_course as course on context.instanceid = course.id and course.theme like 'tdc%'
where context.contextlevel = 50 and ra.roleid = 5
group by cat.id, cat.name, course.id, course.fullname, course.theme
order by cat.name
答案 1 :(得分：0)
通过这种分组方式：GROUP BY ALL。这是cte6答案的替代方法。
调整后的查询：
select
concat('<a target="_blank" href="%%wwwroot%%/course/index.php?categoryid=',cat.id,'">', cat.name,'</a>') as "Category",
concat('<a target="_new" href="%%wwwroot%%/course/view.php?id=',course.id,'">',course.fullname,'</a>') as "Course",
course.theme as "Theme",
count(course.id) as "Students"
from prefix_course as course
join prefix_context as context on context.instanceid = course.id
join prefix_role_assignments as ra on ra.contextid = context.id
join prefix_course_categories as cat on cat.id = course.category
where context.contextlevel = 50 and ra.roleid = 5 and course.theme like 'tdc%'
group by all cat.id, cat.name, course.id, course.fullname, course.theme
order by cat.name
答案 2 :(得分：0)
在解决这个难题之后，我意识到我的问题在于选择执行该任务所需的表。我已经通过以下select语句和表格解决了这个问题。我还使用了不同的伪数据（对不起），所以我用新的伪数据更新了我的OP：
select
concat('<a target="_blank" href="%%wwwroot%%/course/index.php?categoryid=',cat.id,'">', cat.name,'</a>') as "Category",
concat('<a target="_new" href="%%wwwroot%%/course/view.php?id=',c.id,'">',c.fullname,'</a>') as "Course",
c.theme as Theme,
sum(iif(r.id is null,0,1)) as '# of Students'
from prefix_user as u
join prefix_role_assignments as ra on u.id = ra.userid
left outer join prefix_role as r on ra.roleid = r.id and r.id = 5 -- student role id in moodle.
join prefix_context as ct on ra.contextid = ct.id
join prefix_course as c on c.id = ct.instanceid and ct.contextlevel = 50 -- 'course' context type.
join prefix_course_categories as cat on cat.id = c.category and c.theme like 'tdc%'
group by cat.id, c.id, cat.name, c.fullname, c.theme
表格： prefix_course_categories：
| id | name |
|-----|-----------|
| 1 | Baking |
| 2 | Chemistry |
| 3 | IT |
prefix_course：
| id | name | categoryid | theme |
|-----|--------------------|-------------|--------------| -- prefix_course.categoryid =
| 8 | Baking 101 | 1 | tdcemuteal | -- prefix_course_categories.id
| 90 | Web Programming | 2 | NULL |
| 287 | Intro to Excel | 2 | tdcemuaqua |
| 396 | Chemical Reactions | 3 | tdcemuorange |
prefix_user：
| id |
|------|
| 1 |
| 2 |
| 5 |
| 8 |
| 15 |
| 16 |
| 20 |
| 23 |
| 77 |
| 99 |
| 303 |
| 879 |
| 959 |
| 1235 |
prefix_role：
| id | name |
|-----|---------------------|
| 1 | Manager |
| 2 | Course Creator |
| 3 | Teacher |
| 4 | Non-Editing Teacher |
| 5 | Student |
prefix_role_assignments：
| id | contextid | userid | roleid |
|-----|------------|--------|--------| -- prefix_role_assignments.contextid =
| 1 | 1 | 879 | 5 | -- prefix_context.id
| 2 | 2 | 303 | 5 |
| 3 | 3 | 5 | 5 | -- prefix_role_assignments.userid =
| 4 | 4 | 15 | 6 | -- prefix_user.id
| 5 | 5 | 57 | 5 |
| 6 | 6 | 23 | 5 | -- prefix_role_assignments.roleid =
| 7 | 7 | 959 | 5 | -- prefix_role.id
| 8 | 8 | 99 | 5 |
| 9 | 9 | 879 | 5 |
| 10 | 10 | 303 | 5 |
| 11 | 11 | 5 | 5 |
| 12 | 12 | 15 | 5 |
| 13 | 13 | 57 | 5 |
| 14 | 14 | 23 | 5 |
| 15 | 15 | 959 | 5 |
| 16 | 16 | 77 | 5 |
| 17 | 17 | 1235 | 5 |
| 18 | 18 | 8 | 1 |
| 19 | 19 | 23 | 7 |
| 20 | 20 | 287 | 7 |
| 21 | 21 | 287 | 7 |
| 22 | 22 | 287 | 7 |
| 23 | 23 | 287 | 7 |
| 24 | 24 | 699 | 7 |
prefix_context：
| id | contextlevel | instanceid |
|-----|---------------|------------|
| 1 | 50 | 8 | -- prefix_context.instanceid =
| 2 | 50 | 8 | -- prefix_course.id
| 3 | 50 | 8 |
| 4 | 50 | 8 | -- instanceid where contextlevel = 50 are courses that students enrol into.
| 5 | 50 | 8 |
| 6 | 50 | 8 |
| 7 | 50 | 8 |
| 8 | 90 | 8 |
| 9 | 50 | 287 |
| 10 | 50 | 287 |
| 11 | 50 | 287 |
| 12 | 50 | 287 |
| 13 | 50 | 287 |
| 14 | 50 | 287 |
| 15 | 50 | 287 |
| 16 | 50 | 287 |
| 17 | 50 | 287 |
| 18 | 60 | 287 |
| 19 | 50 | 396 |
| 20 | 20 | 287 |
| 21 | 60 | 287 |
| 22 | 10 | 287 |
| 23 | 80 | 287 |
| 24 | 50 | 699 |