尋找失蹤巴黎之SQL

Question

假設有3臺關係數據庫：

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

我想寫一個SQL，讓我說，不存在學生課程對。如果這是不可行的，至少應該知道是否存在缺失對。

此外，由於這是我想自動化的一個較大問題的一小部分，因此查看許多不同的方法可能會很有用。

Answer 1

1找出所有對，然後去除存在對（通過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)