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Question

I am trying to generate an array which contains the number of messages sent from a employee to all (if any or zero if there is no communication) using OPTIONAL MATCH Clause, but its not working. I am not getting zeroes in the array when there is no communication. I am using the following query:

MATCH(e:Employee{key:101,div:'finance'}),(b:Employee)
OPTIONAL MATCH (e)-[r:Message]->(b)
WITH e.name as  em, r.NUMBER_OF_MESSAGES as msg 
ORDER BY msg DESC
RETURN em, COLLECT(msg) AS row

Here is the output:

Row: [31,20,12,10,8,7,7,2,2,2,2,1,1,1]

What am I doing wrong here? Thanks in advance.

Answer 1

The problem here is that collect() will collect all values ignoring nulls . When has no communication between two nodes the value of msg variable will be null and consequently ignored by COLLECT() . To fix it change your query to:

MATCH(e:Employee{key:101,div:'finance'}),(b:Employee)
OPTIONAL MATCH (e)-[r:Message]->(b)
WITH e.name as  em, coalesce(r.NUMBER_OF_MESSAGES,0) as msg 
ORDER BY msg DESC
RETURN em, COLLECT(msg) AS row

This query use the coalesce() function. This function will return the first non-null value in the list of arguments passed to it. Then, if r.NUMBER_OF_MESSAGES is null it will be replaced by zero.