Selecting the two most common attribute pairings from a Entity-Attribute Table? - sql

Question

I have a simple Entity-Attribute table in my database describing simply if an Entity has some Attribute by the existance of a row consisting of (Entity, Attribute).
I want to find out, of all the Entities with two and only two Attributes, what are the most common Attribute pairs
For example, if my table looked like:
+--------+-----------+
| Entity | Attribute |
+--------+-----------+
| Bob | A |
| Sally | B |
| Terry | C |
| Bob | B |
| Sally | A |
| Terry | D |
| Larry | C |
+--------+-----------+
I would want it to return
+-------------+-------------+-------+
| Attribute-1 | Attribute-2 | Count |
+-------------+-------------+-------+
| A | B | 2 |
| C | D | 1 |
+-------------+-------------+-------+
I currently have a short query that looks like:
WITH TwoAtts (
SELECT entity
FROM table
GROUP BY entity
HAVING COUNT(att) = 2
)
SELECT t1.att, t2.att, COUNT(entity)
FROM table t1
JOIN table t2
ON t1.entity = t2.entity
WHERE t1.entity IN (SELECT * FROM TwoAtts)
AND t1.att != t2.att
GROUP BY t1.att, t2.att
ORDER BY COUNT(entity) DESC
but is only capable of producing "duplicate" results like
+-------------+-------------+-------+
| Attribute-1 | Attribute-2 | Count |
+-------------+-------------+-------+
| A | B | 2 |
| B | A | 2 |
| D | C | 1 |
| C | D | 1 |
+-------------+-------------+-------+
In a sense I would like to be able to run a unordered DISTINCT / set operator over the two attribute columns, but I am not sure how to acheive this functionality in SQL?

Answer 1

Hmmm, I think you want two levels of aggregation, with some filtering:
select attribute_1, attribute_2, count(*)
from (select min(ea.attribute) as attribute_1, max(ea.attribute) as attribute_2
from entity_attribute ea
group by entity
having count(*) = 2
) aa
group by attribute_1, attribute_2;
Here is a db<>fiddle