I have tables named course, student and students_in in a MySQL database. The tables look like this:
course
course_id name
3 Physics
12 English
19 Basket Weaving
4 Computer Science
212 Discrete Math
102 Biology
20 Chemistry
50 Robotics
7 Data Engineering
student
id name
2 Sally
1 Bob
17 Robert
9 Pierre
12 Sydney
41 James
22 William
5 Mary
3 Robert
92 Doris
6 Harry
students_in
course_id student_id grade
3 2 B
212 2 A
3 12 A
19 12 C
3 41 A
4 41 B
212 41 F
19 41 A
12 41 B
3 17 C
4 1 A
102 1 D
102 22 A
20 22 A
20 5 B
50 3 A
12 92 B
12 17 C
7 6 A
Here is a Fiddle: 
Since Sally took course IDs 3 and 212, the desired result would look like this (not the colorful table above, which I provided for illustration of the logic involved):
student_id student_name
12 Sydney <-- took course ID 3 with Sally
41 James <-- took course ID 3 and 212 with Sally
17 Robert <-- took course ID 3 with Sally
1 Bob <-- took course ID 4 with James
92 Doris <-- took course ID 12 with James and Robert
102 William <-- took course ID 102 with Bob
I tried to solve this problem by using a Common Table Expression (CTE) to query the first-degree relationships, and can probably use two additional CTEs to get the second-degree and third-degree relationships. But, this feels like a very inelegant way to do this.
Can someone please help with an elegant approach to this problem?
Thank you!
CodePudding user response：
You can use a recursive cte:
with recursive cte(cid, sid, name, l) as (
select si.course_id, si.student_id, s.name, 1 from students_in si
join student s on s.id = si.student_id where si.course_id in (select si1.course_id
from students_in si1 join student s1 on s1.id = si1.student_id and s1.name = 'Sally') and s.name != 'Sally'
union all
select si.course_id, si.student_id, s.name, c.l 1 from cte c
cross join students_in si
join student s on s.id = si.student_id
where si.course_id in (select si1.course_id
from students_in si1 where si1.student_id = c.sid) and si.course_id != c.cid and si.student_id != c.sid and c.l < 3
)
select distinct sid, name from cte where name != 'Sally'
CodePudding user response：
With Recursive coursemates As (
Select y.student_id, 1 as removal
From students_in x Inner Join students_in y
On x.course_id=y.course_id
Where x.student_id=2
UNION
Select y.student_id, r.removal 1
From coursemates r Inner Join students_in x
On r.student_id=x.student_id
Inner Join students_in y
On x.course_id=y.course_id
Where removal<=2
)
Select c.student_id, min(c.removal) as howfar, min(s.name) as student_name
From coursemates c Left Outer Join student s
On c.student_id=s.student_id
Where student_id <> 2
Group By c.student_id
Order by 2, 1
A little verbose, but also a little more generalized than your try in that you can control the depth.
A few defensive additions: 1. Left join on student table in case no R.I. there. 2. Filter out Sally from the result (don't care that Robert was with Sally and then Sally was with Robert)
CodePudding user response：
Join's repeated as many times as needed, also good, but perhaps less elegant:)
with rel as ( --join student->student thru course
select t1.student_id s1Id, t2.student_id s2Id
from students_in t1 inner join students_in t2
on t2.course_id=t1.course_id
where t2.student_id<>t1.student_id
group by t1.student_id,t2.student_id
)
,four as(
select t1.s1Id as s1Id1 ,t2.s1Id s2Id1,t2.s2Id s2Id2 ,t3.s2Id s3Id2
from rel t1 left join rel t2 on t2.s1Id=t1.s2Id and t2.s2Id<>t1.s1Id
left join rel t3 on t3.s1Id=t2.s2Id and t2.s2Id<>t1.s1Id
and t3.s2Id<>t1.s1Id
where t1.s1Id=2
group by t1.s1Id ,t2.s1Id,t2.s2Id,t3.s2Id
)
select t1.s1Id1,t1,s3Id2,s1.name,s3.name,s4.name,s6.name
from four t1
inner join student s1 on t1.s1Id1=s1.id
inner join student s3 on t1.s2Id1=s3.id
inner join student s4 on t1.s2Id2=s4.id
inner join student s6 on t1.s3Id2=s6.id