Question 21212 in Mysql Question posted in
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SQL: elegant way to get first, second, and third degree associations – Mysql

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: http://sqlfiddle.com/#!17/8d86ee/34

My goal is to get the id and name of the students who:

  1. have taken a course with Sally (i.e. "first-degree" relationship), OR
  2. have taken a course with someone who has taken a course with Sally (i.e. "second-degree" relationship), OR
  3. have taken a course with someone who has taken a course with someone who has taken a course with Sally (i.e. "third-relationship" relationship)

Essentially, we’re looking for first-, second-, and third-degree relationships to Sally.

Here is a depiction of what this looks like:

enter image description here

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!

Tags:

3

Answers


  1. 0

    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'

    See fiddle.

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  2. 0 
    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)

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  3. 0

    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
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