Commonality in many to many relationship SQL query - mysql

Question

If I have agents that can have many properties, and vice versa, defined in a in a junction table, how would I query the top agents (an agent who has at least two properties in common with two other agents.)
agent
id
1
2
3
4
5
properties
id
1
2
3
4
5
6
agent_properties
agent_id
property_id
1
1
1
2
1
3
2
2
2
3
3
1
3
3
3
5
4
3
4
4
4
6
5
1
5
2
5
5
6
4
6
6
Example:
Agent 1 (propr1, propr2, propr3),
Agent 2 (propr2, propr3),
Agent 3 (propr1, propr3, propr5)
Agent 4 (propr3, propr4, propr6)
Agent 5 (propr1, propr2, propr5)
Agent 6 (propr4, propr6)
So the query should return Agent 1, Agent 3, Agent 5
If anyone can show how to do this using Laravel Eloquent that would be great.

Answer 1

A solution would be to work with bits.
First you should add a new column in table properties and this new column to be named bin_value. The values from this column must be unique per property and to be power of 2. In your case would be:
id
bin_value
1
1
2
2
3
4
4
8
5
16
6
32
Second, add a new column to agent table named properties and run following update query:
UPDATE agent AS a
INNER JOIN (
SELECT ap.agent_id, SUM(p.bin_value) AS bin_properties
FROM agent_properties AS ap
INNER JOIN properties AS p ON p.id = ap.property_id
GROUP BY ap.agent_id
) AS b ON a.id = b.agent_id
SET a.properties = b.bin_properties
;
Third, run the query with bit function which helps you to find agents which have at least 2 shared properties with at least 2 other agents:
SELECT a1.agent_id
, a1.name
, COUNT(*) AS total_other_agents
FROM agent AS a1
INNER JOIN agent AS a2 ON a1.id != a2.id
AND BIT_COUNT(a1.properties & a2.properties) >= 2 -- at least 2 shared properties
GROUP BY a1.id
HAVING total_other_agents >= 2 -- at least 2 other agents
ORDER BY total_other_agents DESC, a1.name
;
If you have a large number of properties try to set values in bin_value like this:
UPDATE properties
SET bin_value = BINARY(POWER(2, id-1))
;