sql - 如何从一个表中选择一条记录并计算出现在另一个表中的数量并计算它

Question

我有下面的拖表表1：教师表

teacher_id   teacher_name
1            xx
2            yy
3            zz

表 2：学生表

stu_id       stu_name     tearcher1_id     teacher2_id    tearcher3_id
1            aa           1                2 
2            bb           2                3
3            cc           1

我想从老师那里得到一个 sql 语句的列表，包括出现在学生表中的人数，如下所示：

teacher_id      teacher_name          num_selected_by_stu
1               xx                    2
2               yy                    2
3               zz                    1

我在sql下面试过但似乎不起作用，

    select * from teatcher t1 
    left join (
    select stu_id,tearcher1_id,tearcher2_id,tearcher3_id,count(stu_id) as num_selected_by_stu from student 
    group by stu_id,tearcher1_id,tearcher2_id,tearcher3_id) t2 
    ON ( t2.teacher1_id=t1.teacher_id or t2.teacher2_id=t1.teacher_id or t2.teacher3_id=t1.teacher_id)

而且，有人可以帮忙吗？

Answer 1

SELECT 
  teacher_id
  teacher_name
  NVL(num1, 0) + NVL(num2, 0) + NVL(num3,0) as num_selected_by_stu
FROM 
  teacher t
  left outer join ( SELECT count(*) as num1, tearcher1_id FROM student group by tearcher1_id ) t1 on t1.tearcher1_id = t.tearcher_id
  left outer join ( SELECT count(*) as num2, tearcher2_id FROM student group by tearcher2_id ) t2 on t2.tearcher2_id = t.tearcher_id
  left outer join ( SELECT count(*) as num3, tearcher3_id FROM student group by tearcher3_id ) t3 on t3.tearcher3_id = t.tearcher_id
;

Answer 2

请试试：

SELECT 
    *,
    (SELECT 
        COUNT(*) 
    FROM 
        Student t2 
    WHERE 
        t1.teacher_id=t2.tearcher1_id OR 
        t1.teacher_id=t2.teacher2_id OR 
        t1.teacher_id=t2.tearcher3_id) num_selected_by_stu
FROM 
    Teacher t1

Answer 3

select t.teacher_id,t.teacher_name from teacher t ,

inner join student s
 on t.teacher_id = s.teacher_id
 where 
 case when teacher1_id < 35  then 0 else 1 end +
 case when teacher1_id < 35  then 0 else 1 end +
 case when teacher1_id < 35  then 0 else 1 end +
 end 
 as [num_selected_by_stu]