Check if node exists - proposal for a new APOC-function

[janezic]

Hello,

I would like to suggest that in the next version of APOC a new function could be implemented which checks, if a certain node with a specific value in a property exists or not.

For example:

apoc.node.exist(u:User {user_email: "abc@xyz.at"})

The result could be a very simple TRUE / FALSE.

Thank you,

JJJ

[michael_hunger]

Why not just use cypher matching? Or existential subqueries?

WHERE exists { MATCH (u:User {user_email: "abc@xyz.at"}) }

[janezic]

Thank you Michael, two users of the community (I don't know how to mention them here) have already found a solution in another thread (I don't know how to link there).

[clem]

It was in this thread:

Check if node exists and use the result as a condition Cypher

I added a suggestion to the documentation for OPTIONAL MATCH. This is a great example! I've learned a lot! Thanks for showing us OPTIONAL MATCH. I hadn't quite appreciated what it could do.

[clem]

I think it's a little trickier than that.

This does NOT work (although it seems like it should):
RETURN exists (MATCH (u:User {name: "abc@xyz.at"}))

Invalid input ')': expected whitespace or a relationship pattern (line 1, column 55 (offset: 54))
"RETURN  exists (MATCH (u:User {name: "abc@xyz.at"}))"
                                                       ^

In other words, this suggestion would be satisfied if exists()function was expanded to take a larger types of arguments.

[mrksph]

Hi,
Just tried this and it didn't work.

Got the following error:

Invalid input 'WHERE': expected 
  "RETURN"
  "CREATE"
  "DELETE"
  "SET"
  "REMOVE"
  "DETACH"
  "MATCH"
  "WITH"
  "UNWIND"
  "USE"
  "CALL"
  "LOAD"
  "FROM"
  "FOREACH"
  "MERGE"
  "OPTIONAL"
  "USING" (line 1, column 1 (offset: 0))
"WHERE exists { MATCH (u:Client{code:"12"}) }"
 ^

Neo4j 4.2.1

Thanks

[michael_hunger]

Perhaps you need 4.3 for that but I thought it would work in 4.2 already

just tried this on 4.3

WITH 1 as x WHERE exists { MATCH (u:Client{code:"12"}) } RETURN *

[michael_hunger]

Curly braces. Not parentheses

[janezic]

Michael, could you please outline the difference between exists() and exists {}?

THX, JJJ

[michael_hunger]

exits() is a predicate function, which might be deprecated at some point.

And exists {} is more like a structural clause, also called existential subquery, so like a subquery but one that only returns false/true on zero/non-zero results of the query (which currently can contain match/where)

Neo4j Graph Database Platform

WHERE - Neo4j Cypher Manual

`WHERE` adds constraints to the patterns in a `MATCH` or `OPTIONAL MATCH` clause or filters the results of a `WITH` clause.

[clem]

@michael.hunger

Unfortunately, I'm at the stage of where Cypher is like Legos: I can build fragments but I can't always build an entire structure that I want. I'm beginning to understand subqueries (which I believe don't get enough emphasis) but I'm not fully there... I don't understand why I'm not allowed to put certain pieces together in certain ways that I would have thought possible.

I naively tried this:

RETURN exists { MATCH (u:User {name: "abc@xyz.at"}) }

but I got this error msg...

The EXISTS subclause is not valid inside a WITH or RETURN clause. (line 1, column 9 (offset: 8))
"RETURN exists {MATCH (u:User {name: "abc@xyz.at"})}"
^

I do see the exist subquery documentation, but it doesn't match up with what I (and the OP) would like to do.

Neo4j Graph Database Platform

Subqueries - Developer Guides

Building on the previous Cypher guides, this guide shows how to write subqueries. Upon finishing this guide, you should be able to write and understand queries using this capability.

I also tried this: I'd like to do an existing test within apoc.when but this doesn't seem to work either. It seems that Cypher is insisting on having a relationship pattern but I just want to test for existence of a node with a certain property's value.

RETURN
apoc.do.when(exists {(u:User {name: "abc@xyz.at"})}, "RETURN true", "RETURN false") 

Invalid input '}': expected whitespace or a relationship pattern (line 2, column 67 (offset: 73))
"apoc.do.when(exists {MATCH(n) WHERE (u:User {name: "abc@xyz.at"})}, "RETURN true", "RETURN false")"

We did come up with a few alternatives to using EXISTS of a property here:

Check if node exists and use the result as a condition Cypher

Sorry, to ask this question for the 100th time, but none of the previous posts was similar to my problem: I just want to get back TRUE or FALSE if a node with a certain label and a certain value of a property exists. (u:User {user_id: 1} => TRUE / FALSE All other posts where about MERGE and so on, but I don't want to create anything, I want to use it as a condition in an apoc.do.when-function.

The documentation for exists() function doesn't have a pointer to exists subqueries:

Neo4j Graph Database Platform

Predicate functions - Neo4j Cypher Manual

Predicates are boolean functions that return `true` or `false` for a given set of non-`null` input.

Sorry for my confusion... I do think if there was an easy way to test for property-with-a-value existence, it would be helpful.