Question

Lets say we have the table SP(sid, pid) which shows student sid is involved in project pid.
Now I want a mysql query to select all the pairs of students who involved at least in one project, and the number of projects that they collaborate in.

For example:
input:
S1    P1
S1    P2
S2    P1
S3    P1
S4    P1
S4    P2

Output:

S1   S2  1
S1   S3  1
S1   S4  2

S2   S3  1
S2   S4  1
S3   S4  1

Answer 1

Eric’s answer might be a bit improved:

select
    sp1.sid as sid1,
    sp2.sid as sid2,
    count(*) as num
from
    sp as sp1
    inner join sp as sp2 on sp1.pid=sp2.pid and sp1.sid<sp2.sid
group by sp1.sid, sp2.sid
;

Use < instead of <> to avoid geting (1,4,2) AND (4,1,2)