Question

我有以下两种模式：

advisor(s_ID, i_ID)
instructor(ID, name, dept_name, credits)

我已按以下方式计算了每个不同i_ID的i_ID出现次数：

SELECT i_ID, count(*)
FROM advisor
GROUP BY i_ID;

每个i_ID都属于dept_name。如何获得每个部门名称的总计数？

示例：

i_ID     count 
21         3
23         4
27         1
29         5

假设i_ID 21,23属于讲师（ID，物理），i_ID 27,29属于讲师（ID，化学），那么输出将是：

dept_name   count
physics       7(3+4)
chemistry     6(1+5)

Answer 1

我希望这会有所帮助。

select dept name, 0 as total advised
from (select dept name from department
minus
select dept name
from instructor)
union
select dept name, count(*) as total advised
from instructor, advisor
where instructor.id = advisor.I ID
group by dept name
order by dept name;

Answer 2

试试这个

  select a.dept_name,count(*) as count from instructor a join advisor b
    on a.ID=b.i_ID group by  a.ID

如何将一组关系从一个关系转换为另一个关系？

2 个答案: