Question

我对我正在尝试运行的sql代码有些怀疑。首先，我有2个表格没有讨论：

当然

id_course
name_course
date_begin
date_finish
number_students_enrolled

students_course

id_student
id_course
状态

我需要执行“课程”表的选择，但我必须计算“students_course”中的行，如果它对应于同一个课程并且状态等于3，则将结果添加到“number_students_enrolled”列中我不知道如何组合select语句并将结果添加到第一个表中。

示例，我有一个课程，有9名学生注册，但在“students_course”表中存在另外3行属于同一课程ID，并且状态等于3，所以我必须计算它然后添加它到number_students_enrolled列并显示最终结果，在这种情况下它将是12.你能帮助我尝试解决这个问题吗？谢谢！

一些示例数据：

当然

id_course - name_course - date_begin - date_finish - number_students_enrolled
  150     -    maths    -   02/10/16 -   04/10/16  -          9
  172     -    economy  -   02/14/16 -   05/08/16  -          21

students_course

id_student - id_course - state
  10       -    150    -  3
  12       -    150    -  3
  13       -    150    -  3
  14       -    150    -  1
  15       -    194    -  2
  16       -    194    -  3

选择后我必须获得以下结果

id_course - name_course - date_begin - date_finish - number_students_enrolled
  150     -    maths    -   02/10/16 -   04/10/16  -          12
  172     -    economy  -   02/14/16 -   05/08/16  -          21

Answer 1

这应该适合你：

SELECT c.name_course,c.number_students_enrolled+(SELECT count(sc2.id_student)
FROM dbo.students_course sc2    WHERE sc2.id_course=c.id_course 
AND sc2.state   =3) FROM dbo.course c

Answer 2

不考虑您已经有 number_students_enrolled 列，我正在给您一个查询。希望它有所帮助。

  select C.id_course, C.name_course, 
         (select count(1) 
            from students_course 
           where id_course = C.ID 
             and state = 3) as call_as_you_need_this_column
    from course C
order by C.id_course

这只是达到你需要的一种方式。

Answer 3

此查询将为您提供已注册学生的数量：

select c.id_course, count(*) as students_enrolled
from course c join student_course sc on c.id_course = sc.id_course
group by id_course;

example

如果你想要“预先计算，我会建议一个观点。”

Answer 4

尝试此查询以获取结果

  SELECT c.id_course,c.name_course,c.date_begin,c.date_finish,
   (NVL(temp.cnt, 0)+c.number_students_enrolled)
   FROM course c   
   left join (SELECT count(sc.id_student) as cnt,sc.id_course as course_id
   FROM students_course sc  WHERE sc.state   =3 group by sc.id_course ) as temp 
   on temp.course_id =c.id_course

SQL将select作为列

4 个答案: