Question

假设有一个包含3个表的关系数据库：

Courses {name, id},
Students {name, id},
Student_Course {student_id, course_id}

我想编写一个SQL，它为我提供了不存在的学生课程对。如果这不可行，至少知道是否缺少对是很好的。

另外，由于这是我想要自动化的一个较大问题的一小部分，因此看到许多不同的方法会很有用。

Answer 1

首先查找所有对，然后删除当前对（通过left join/not null或not exists）

select s.id as student_id, c.id as course_id
from Courses as c
cross join Students as s
left join Student_Course as sc on sc.student_id = s.id and sc.course_id = c.id
where sc.course_id is null -- any sc field defined as "not null"

Answer 2

with Courses as(
select 1 as id,'Math' as name union all
select 2 as id,'English' as name union all
select 3 as id,'Physics' as name union all
select 4 as id,'Chemistry' as name),
Students as(
select 1 as id,'John' as name union all
select 2 as id,'Joseph' as name union all
select 3 as id,'George' as name union all
select 4 as id,'Michael' as name
),
studcrse as(
select 1 as studid, 1 as crseid union all
select 1 as studid, 2 as crseid union all
select 1 as studid, 3 as crseid union all
select 2 as studid, 3 as crseid union all
select 2 as studid, 4 as crseid union all
select 3 as studid, 1 as crseid union all
select 3 as studid, 2 as crseid union all
select 3 as studid, 4 as crseid union all
select 3 as studid, 3 as crseid union all
select 4 as studid, 4 as crseid )

SELECT A.ID AS studentId,a.name as studentname,b.id as crseid,b.name as crsename
from Students as a 
cross join
Courses as b
where not exists
(
select 1 from studcrse as c
where c.studid=a.id
and c.crseid=b.id)

在SQL中找到丢失的对

2 个答案: