- Advanced Join > X,Y pairs. **
- where conditions can be used in having too
https://www.hackerrank.com/challenges/symmetric-pairs/problem?isFullScreen=true
select t1.x x1, t1.y y1
from functions t1
inner join functions t2
on t1.x = t2.y and t1.y = t2.x
group by t1.x, t1.y
having 1=1
and count(*) > 1 -- if count = 1, it is self symmetric(x=y). There should be more than one self symmetric pairs
or t1.x < t1.y -- for those symmetric pairs where x <> y
order by t1.x
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