How to query symmetric graph and return non duplicate rows

[d_mulyalin]

Hi,

Run into the issue where Cypher returns two rows per my query per each path.

Here are Cypher statements to demonstrate my issue:

//Create nodes and relationships:
CREATE (:Person { Name: "Max"})
CREATE (:Person { Name: "Sam"})
CREATE (:Person { Name: "Nil"})

MATCH (a:Person) where a.Name="Max"
MATCH (b:Person) where b.Name="Sam"
MATCH (c:Person) where c.Name="Nil"
CREATE (a)-[:KNOWS]->(b)
CREATE (a)-[:KNOWS]->(c)

//Graph now looks like this:
(Sam)<-[KNOWS]-(Max)-[KNOWS]->(Nil)

//Run query:
MATCH (p1:Person)<--(:Person)-->(p2:Person)
RETURN p1.Name, p2.Name

//Results I get
p1.Name p2.Name
"Nil" "Sam"
"Sam" "Nil"

While desired results would be:
p1.Name p2.Name
"Nil" "Sam"

or
p1.Name p2.Name
"Sam" "Nil"

How do I instruct Cypher to traverse this symmetric path only once and return one row in results instead of two.

[Thomas_Silkjaer]

Add a condition? WHERE p1 <> p2

[adam_cowley]

You could add a where clause to compare the two nodes and only return one when a property is higher than the other - either by property, internal id or just simply:

WHERE p1 > p2

[d_mulyalin]

Thomas_Silkjaer:

WHERE p1 <> p2

Tried this:
MATCH (p1:Person)<--(:Person)-->(p2:Person)
WHERE p1 <> p2
RETURN p1.Name, p2.Name

Returns:

p1.Name	p2.Name
"Nil"	"Sam"
"Sam"	"Nil"

[d_mulyalin]

That one works, tried:
MATCH (p1:Person)<--(:Person)-->(p2:Person)
WHERE id(p1) > id(p2)
RETURN p1.Name, p2.Name

Returns:

p1.Name	p2.Name
"Nil"	"Sam"

Are there any more "natural" way to get desired results similar to what "distinct" clause does but on a path level?

At the same time, running this query:
MATCH (p1:Person)<--(:Person)-->(p2:Person)
WHERE p1 > p2
RETURN p1.Name, p2.Name

Returns Error:
Type mismatch: expected Float, Integer, Point, String, Date, Time, LocalTime, LocalDateTime or DateTime but was Node (line 2, column 7 (offset: 50))
"WHERE p1 > p2"

[andrew_bowman]

The recommended approach shared is the best way at this point.

Order of nodes matters in a path, and so having two symmetrical path results, where only the order is different, means the paths are distinct from each other already.

That said, this is a fairly common restriction in queries, and a keyword or more concise means to express this restriction sounds reasonable.

I'll add this as a feature consideration.