Question

我有3张桌子：钱，学生，教师。该查询返回每个系中的每个系和最高津贴。

select 
    f.name as "FACULTY_NAME",
    max(stipend) as "MAX_STIPEND"
from 
    money m, student s 
inner join
    faculty f on f.id_faculty = s.faculty_id
where 
    m.student_id = s.id_student
group by 
    f.id_faculty, f.name;

查询工作正常：

FACULTY_NAME     |    MAX_STIPEND
-----------------+---------------
IT Faculty       |    50
Architecture     |    60
Journalism       |    40

但是，当我在原始查询中添加s.name来显示接收max_stipend的学生的姓名时，查询也无法像以前那样工作-它会返回所有学生

select 
    f.name as "FACULTY_NAME",s.name,
    max(stipend) as "MAX_STIPEND"
from 
    money m, student s 
inner join
    faculty f on f.id_faculty = s.faculty_id
where 
    m.student_id = s.id_student
group by 
    f.id_faculty, f.name, s.name;

查询结果：

FACULTY_NAME    |   s.name  |   MAX_STIPEND
----------------+-----------+---------------
IT Faculty      |   Joe     |   50
IT Faculty      |   Lisa    |   10
Architecture    |   Bob     |   60
Journalism      |   Fred    |   5
Architecture    |   Susan   |   5
Journalism      |   Tom     |   40

使用右，左和内联接执行相同的操作。有人可以告诉问题出在哪里吗？

Answer 1

首先，您应该对所有所有联接使用正确的JOIN语法。

第二，您可以使用Oracle的keep语法：

select f.name as FACULTY_NAME,
       max(stipend) as MAX_STIPEND,
       max(s.name) keep (dense_rank first order by stipend desc)
from money m join
     student s 
     on  m.student_id = s.id_student join
     faculty f
     on f.id_faculty = s.faculty_id   
group by f.id_faculty, f.name;

Answer 2

但是，当我在原始查询中添加s.name来显示接收max_stipend的学生的姓名时，查询也无法像以前那样工作-它会返回所有学生

添加s.name时，您正在寻找每个用户的最小值。

如果您想要拥有MAX_STIPEND的用户名，则应转到窗口功能。例如，MS SQL Server中的Dense Rank。

with cte as
(select 
    f.name as "FACULTY_NAME",
    s.name as "STUDENT_NAME",
    stipend as "MAX_STIPEND",
    DENSE_RANK() OVER   
    (PARTITION BY f.name, s.name ORDER BY i.stipend DESC) AS Rank  
 from 
    money m 
 inner join student s on m.student_id = s.id_student
 inner join
    faculty f on f.id_faculty = s.faculty_id
)
select "FACULTY_NAME", "STUDENT_NAME"
from cte
where rank = 1

并非所有的sql品牌都具有窗口功能。这里的dense_rank on MySQL以及dense_Rank for Oracle

的链接

最大函数返回多个值[SQL]

2 个答案: