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Does Mempgrah support something like multiple matches? I keep running into an issue that produces result 0. I have several queries, when I run them one by one, I get some results back, but if I arrange them as multiple matches I end up with zero.

My code is:

MATCH (e:Employee {status:'initiated', id:'id-1'}), (o:Office {status:'initiated', id:'id-1'})
WITH COLLECT(e) + COLLECT(o) AS allEntities
MATCH (d:Gadget {status:'initiated', id:'id-1'}), (a:Alarm {status:'initiated', id:'id-1'})
WITH allEntities, COLLECT(d) + COLLECT(a) AS allDevices
RETURN size(allEntities)//, size(allDevices);
Elwood Kharson
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1 Answers1

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Could it be possible that one of your MATCH queries returns an empty set of results? Multiple MATCH clauses behave like a cartesian product and therefore return null if any of the results are an empty set.

You can test this by running separate queries:

MATCH (e:Employee {status:'initiated', id:'id-1'}), (o:Office {status:'initiated', id:'id-1'})
WITH COLLECT(e) + COLLECT(o) AS allEntities
RETURN size(allEntities);

and then:

MATCH (d:Gadget {status:'initiated', id:'id-1'}), (a:Alarm {status:'initiated', id:'id-1'})
WITH COLLECT(d) + COLLECT(a) AS allDevices
RETURN size(allDevices);

If any of these queries returns 0 than that is probably your issue. You can fix this by adding OPTIONAL before MATCH. When using OPTIONAL MATCH empty results are allowed and are not affecting your end result.

MPesi
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