How to find the two combinations of elements that are present the most in all the groups? [closed]

For example in this simple table:

Group	Element
xxx	a
xxx	b
xxx	c
xxx	d
xxx	e
yyy	a
yyy	b
yyy	f
yyy	g
zzz	a
zzz	b
zzz	c
zzz	g

‘a’ and ‘b’ are the combination of elements present the most in all groups

Answer

You can do it with a triangle join

with c1 as (
  select t1.grp, t1.Element e1, t2.Element e2
  from tbl t1
  join tbl t2 on t1.grp = t2.grp and t1.Element < t2.Element 
)
  select e1,e2
  from c1
  group by e1,e2
  order by count(*) desc
  limit 1