How to use max() on a collection? - neo4j

I have a dataset that looks something like this:
CREATE (n {name:'main', val:3}) -[:r]-> ({name:'sub1', val:2}), (n)-[:r]->({name:'sub2', val:1})
Now, I need to find the maximum value for 'val' for all nodes that are connected to the node named 'main' (including 'main' too). So, in this case the answer is
3.
Since the node named 'main' may not have any subnodes, I used OPTIONAL MATCH to find the subnodes, then combine all the vals found into a list and call max() on it, like so:
MATCH (n {name:'main'})
OPTIONAL MATCH (n)-[:r]->(subs)
RETURN max(n.val + collect(subs.val))
But this gives the following error:
Type mismatch: expected Float or Integer but was Collection (line
3, column 18 (offset: 73)) "RETURN max(n.val + collect(subs.val))"
What is the correct may to solve this sort of problem?

This should work for you:
MATCH p=(n {name:'main'})-[:r*0..]->(subs)
UNWIND NODES(p) AS node
RETURN MAX(node.val) AS result;
This query uses a variable-length pattern to follow the entire (optional) chain of r relationships. Your query did not. The NODES() function produces a collection of path nodes, and the UNWIND converts the collection to data rows. Aggregation functions like MAX() only aggregate across data rows.

Related

Returning multiple columns

Hi All newbie here in Neo4J,
I am trying to return keys or properties using the following simple query in the neo4J broswer environment.
MATCH (n:movies),(m:people)
RETURN properties(n,m)
What I am trying to achieve is to return the properties for both the movies and people nodes
However, I would always get an error
Too many parameters for function 'properties' (line 2, column 9 (offset: 36))
" RETURN properties(n,m)"
I have tried,
MATCH (n:movies),(m:people)
RETURN properties(k) in [n,m]
The error I would get
Variable `k` not defined (line 2, column 20 (offset: 47))
" RETURN properties(k) in [n,m]"
I am trying to pass a list here into k but NEO4J is not permitting me to do so. Is it even possible to pass a list into the function properties() ??
Thank you in advance.
The properties function takes exactly one node or a relationship as input.
MATCH (n:movies),(m:people) RETURN properties(n), properties(m)
will create a Cartesian Product.
i.e. If you have five movies and ten people, you will get a result of all 50 combinations.
If you aren't looking for a cartesian product, you would have to define a specific pattern or restrict the MATCH clause further.
If you want just the individual properties without combining them, consider Union.
MATCH (n:Movie)
RETURN properties(n) as `Properties`
UNION ALL
MATCH (m:Person)
RETURN properties(m) as `Properties`
Why am I using aliases for a seemingly simple query? To avoid this:
All sub queries in an UNION must have the same column names (line 3,
column 1 (offset: 39))
For working with lists:
The collect function lets you create/construct a list from the results while UNWIND expands a list into a sequence of rows.
properties() only takes one argument, you can try
MATCH (n:movies),(m:people) RETURN properties(n) as prop_n, properties(m) as prop_m
or more optimal query would be
MATCH (n:movies) optional match (m:people) RETURN properties(n) as prop_n, properties(m) as prop_m
MATCH (n:movies),(m:people)
RETURN properties(k) in [n,m]
since you have not defined k so you are getting the error. Also according to doc properites() takes "An expression that returns a relationship, a node, or a map" as an argument. Your query is not supported.

Collection in WITH clause gets expanded to one element per row

I want to create a map projection with node properties and some additional information.
Also I want to collect some ids in a collection and use this later in the query to filter out nodes (where ID(n) in ids...).
The map projection is created in an apoc call which includes several union matches.
call apoc.cypher.run('MATCH (n)-[:IS_A]->({name: "User"}) MATCH (add)-[:IS_A]->({name: "AdditionalInformationForUser"}) RETURN n{.*, info: collect(add.name), id: ID(n)} as nodeWithInfo UNION MATCH (n)-[:IS_A]->({Department}) MATCH (add)-[:IS_A]->({"AdditionalInformationForDepartment"}) RETURN n{.*, info: collect(add.name), id: ID(n)} as nodeWithInfo', NULL) YIELD value
WITH (value.nodeWithInfo) AS nodeWithInfo
WITH collect(nodeWithInfo.id) as nodesWithAdditionalInfosIds, nodeWithInfo
MATCH (n)-[:has]->({"Vacation"})
MATCH (u)-[:is]->({"Out of Order"})
WHERE ID(n) in nodesWithAdditionalInfosIds and ID(u) in nodesWithAdditionalInfosIds
return n, u, nodeWithInfo
This does not return anything because, when the where part is evaluated it doesn´t check "nodesWithAdditionalInfosIds" as a flat list but instead only gets one id per row.
The problem only exists because I am passing the ids (nodesWithAdditionalInfosIds) AND the nodeProjection (nodeWithInfo) on in the WITH clause.
If I instead only use the id collection and don´t use the nodeWithInfo projection the following adjustement works and returns my only the nodes which ids are in the id collection:
...
WITH collect(nodeWithInfo.id) as nodesWithAdditionalInfosIds
MATCH (n)-[:has]->({"Urlaub"})
MATCH (u)-[:is]->({"Out of Order"})
WHERE ID(n) in nodesWithAdditionalInfosIds and ID(u) in nodesWithAdditionalInfosIds
return n, u
If I just return the collection "nodesWithAdditionalInfosIds" directly after the WITH clause in both examples this gets obvious. Since the first one generates a flat list in one result row and the second one gives me one id per row.
I have the feeling that I am missing a crucial knowledge about neo4js With clause.
Is there a way I can pass on my listOfIds and use it as a flat list without the need to have an exclusive WITH clause for the collection?
edit:
Right now I am using the following workaround:
After I do the check on the ID of "n" and "u" I don´t return but instead keep the filtered "n" and "u" nodes and start a second apoc call that returns "nodeWithInfo" like before.
WITH n, u
call apoc.cypher.run('MATCH (n)-[:IS_A]->({name: "User"}) MATCH (add)-[:IS_A]->({name: "AdditionalInformationForUser"}) RETURN n{.*, info: collect(add.name), id: ID(n)} as nodeWithInfo UNION MATCH (n)-[:IS_A]->({Department}) MATCH (add)-[:IS_A]->({"AdditionalInformationForDepartment"}) RETURN n{.*, info: collect(add.name), id: ID(n)} as nodeWithInfo', NULL) YIELD value
WITH (value.nodeWithInfo) AS nodeWithInfo, n, u
WHERE nodeWithInfo.id = ID(n) OR nodeWithInfo.id = ID(u)
RETURN nodeWithInfo, n, u
This way I can return the nodes n, u and the additional information (to one of the nodes) per row. But I am sure there must be a better way.
I know ids in neo4j have to be used with care, if at all. In this case I only need them to be valid inside this query, so it doesn´t matter if the next time the same node has another id.
The problem is stripped down to the core problem (in my opinion), the original query is a little bigger with several UNION MATCH inside apoc and the actual match on nodes which ids are contained in my collection is checking for some more restrictions instead of asking for any node.
Aggregating functions, like COLLECT(), aggregate over a set of "grouping keys".
In the following clause:
WITH collect(nodeWithInfo.id) as nodesWithAdditionalInfosIds, nodeWithInfo
the grouping key is nodeWithInfo. Therefore, each nodesWithAdditionalInfosIds would always be a list containing one value.
And in the following clause:
WITH collect(nodeWithInfo.id) as nodesWithAdditionalInfosIds
there is no grouping key. Therefore, in this situation, nodesWithAdditionalInfosIds will contain all the nodeWithInfo.id values.

Neo4j indices slow when querying across 2 labels

I've got a graph where each node has label either A or B, and an index on the id property for each label:
CREATE INDEX ON :A(id);
CREATE INDEX ON :B(id);
In this graph, I want to find the node(s) with id "42", but I don't know a-priori the label. To do this I am executing the following query:
MATCH (n {id:"42"}) WHERE (n:A OR n:B) RETURN n;
But this query takes 6 seconds to complete. However, doing either of:
MATCH (n:A {id:"42"}) RETURN n;
MATCH (n:B {id:"42"}) RETURN n;
Takes only ~10ms.
Am I not formulating my query correctly? What is the right way to formulate it so that it takes advantage of the installed indices?
Here is one way to use both indices. result will be a collection of matching nodes.
OPTIONAL MATCH (a:B {id:"42"})
OPTIONAL MATCH (b:A {id:"42"})
RETURN
(CASE WHEN a IS NULL THEN [] ELSE [a] END) +
(CASE WHEN b IS NULL THEN [] ELSE [b] END)
AS result;
You should use PROFILE to verify that the execution plan for your neo4j environment uses the NodeIndexSeek operation for both OPTIONAL MATCH clauses. If not, you can use the USING INDEX clause to give a hint to Cypher.
You should use UNION to make sure that both indexes are used. In your question you almost had the answer.
MATCH (n:A {id:"42"}) RETURN n
UNION
MATCH (n:B {id:"42"}) RETURN n
;
This will work. To check your query use profile or explain before your query statement to check if the indexes are used .
Indexes are formed and and used via a node label and property, and to use them you need to form your query the same way. That means queries w/out a label will scan all nodes with the results you got.

Cypher Access Nodes of Variable Length Path

I have the following problem:
I have a MATCH clause like this one:
MACTH (A)-[*1..]->(B)
Now, if this pattern matches, I need to somehow duplicate the -[*1..]-> part and replace all node in it tat have a specific type with nodes of another type.
for example if I had this graph:
(X1:A)-[:r]->(X2:B)-[:r]->(X3:E)-[:r]->(X4:A)
I might want to get this graph after applying a rule MACTH (A:A)-[*1..]->(B:A) something something
(X1:A)-[:r]->(X2:B)-[:r]->(X3:E)-[:r]->(X4:A) , (X1)-[:r]->(X5:C)-[:r]->(X3)-[:r]->(X4)
by duplicating the path between (A) and (B) and replacing all nodes of type B with type C
But how can I access the -[*1..]-> part of the query?
Try this
Match (p:Person)-[:RELATED*1..]->(e)
with collect(distinct(id(e))) as ids Match (c:Country) where id(c) in ids
remove c:Country set c:NewLabel
return *
on data
Create (p:Person)-[:RELATED]->(b:Person)-[:RELATED]->(c:Client)-[:RELATED]->(e:Country) return *

Reusing path in multiple MATCH UNION queries cypher neo4j

I'd like to pull and combine data from several different paths that share a path at the beginning, not all of which might exist. For example, I'd like to do something like this:
MATCH (:Complex)-[:PATH]->(s:Somewhere)-[:FETCHING]->(data)
RETURN data.attribute
UNION ALL
MATCH (s)-[:OPTIONAL]->(o:OtherData)
RETURN o.attribute;
so that it doesn't retrace the path up to s. I can't actually do this, though, because UNION separates queries and the (s)-[:OPTIONAL] in the second part will match anything with an outgoing OPTIONAL relation; the s is a loose handle.
Is there a better way of doing this than repeating the path:
MATCH (:Complex)-[:PATH]->(s:Somewhere)-[:FETCHING]->(data)
RETURN data.attribute
UNION ALL
MATCH (:Complex)-[:PATH]->(s:Somewhere)-[:OPTIONAL]->(o:OtherData)
RETURN o.attribute;
I made a few attempts using WITH, but they all either caused the query to return nothing if any part failed, or I could not get them to line up into a single column and instead got rows with redundant data, or (with multiple, nested WITHs, which I'm not sure about the scoping of) just fetching everything.
Have you looked at the semantics of an optional match? So you can match to s, beyond s and your optional component. Something like:
MATCH (:Complex)-[:PATH]->(s:Somewhere)
MATCH (s)-[:FETCHING]->(data)
OPTIONAL MATCH (s)-[:OPTIONAL]->(otherData)
RETURN data.attribute, otherData.attribute
Sorry I missed the importance of a single column, is it really important?
You can gather the vaues into a single collection :
MATCH (:Complex)-[:PATH]->(s:Somewhere)
MATCH (s)-[:FETCHING]->(data)
OPTIONAL MATCH (s)-[:OPTIONAL]->(otherData)
RETURN [data.attribute] + COLLECT(otherData.attribute)
But doesn't this work for a single column:
MATCH (:Complex)-[:PATH]->(s:Somewhere)
MATCH (s)-[:FETCHING]->(data)
OPTIONAL MATCH (s)-[:OPTIONAL]->(otherData)
WITH [data.attribute] + COLLECT(otherData.attribute) as col
RETURN UNWIND col AS val

Resources