Question

I have a cypher query like this.

START dep=node:cities(city_code = "JGS"), 
arr=node:cities(city_code = "XMN") 

MATCH dep-[way:BRANCH2BRANCH_AIRWAY*0..1]->()-->arr

RETURN length(way), transfer.city_code,
extract(w in way: w.min_consume_time) AS consumeTime

The relationship named “way” is a optional one, so the property named “consumeTime” will be a empty list when the relationship “way” not exsit.

The query result is:

| 0 | “JGS” | [] |
| 1 | “SZX” | [3600] |

When I want to use the head function with the property “consumeTime”, it return a error “Invalid query: head of empty list”.

How can I get a result like this?

| 0 | “JGS” | null |
| 1 | “SZX” | 3600 |

Answer 1

This would be trivial with conditional expressions, which is something I think is important to add to Cypher: https://github.com/neo4j/community/issues/899

Here’s a somewhat hacky query using reduce for you that requires 1.9-SNAPSHOT:

START dep=node:cities(city_code = "JGS"), 
      arr=node:cities(city_code = "XMN") 
MATCH dep-[way:BRANCH2BRANCH_AIRWAY*0..1]->()-->arr
WITH  length(way) as wayLength, 
      transfer.city_code as transferCityCode,
      extract(w in way: w.min_consume_time) AS consumeTime
WITH  wayLength,
      transferCityCode,
      consumeTime,
      // reverse the consumeTime list, so that we can get the head from the end
      reduce(acc=[], x in consumeTime: x + acc) reverseConsumeTime
RETURN wayLength,
       transferCityCode,
       consumeTime,
       // return null if empty, else the end of the list
       reduce(acc=null, x in reverseConsumeTime: x) as headOrNull;

This query could be improved a fair amount with the following syntax to reverse a collection:
reverse(coll) or conditional expressions to check for an empty list.