Difference between START n = node(*) and MATCH (n) - neo4j

Question

In Neo4j 2.0 this query:
MATCH (n) WHERE n.username = 'blevine'
OPTIONAL MATCH n-[:Person]->person
OPTIONAL MATCH n-[:UserLink]->role
RETURN n AS user,person,collect(role) AS roles
returns different results than this query:
START n = node(*) WHERE n.username = 'blevine'
OPTIONAL MATCH n-[:Person]->person
OPTIONAL MATCH n-[:UserLink]->role
RETURN n AS user,person,collect(role) AS roles
The first query works as expected returning a single Node for 'blevine' and the associated Nodes mentioned in the OPTIONAL MATCH clauses. The second query returns many more Nodes which do not even have a username property. I realize that start n = node(*) is not recommended and that START is not even required in 2.0. But the second form (with OPTIONAL MATCH replaced with question marks on the relationship type) worked prior to 2.0. In the second form, why is 'n' not being constrained to the single 'blevine' node by the first WHERE clause?

Answer 1

To run the second query as expected you would just need to add WITH n. In your query you would need to filter the result and pass it for optional match which is to be done using WITH
START n = node(*) WHERE n.username = 'blevine'
WITH n
OPTIONAL MATCH n-[:Person]->person
OPTIONAL MATCH n-[:UserLink]->role
RETURN n AS user,person,collect(role) AS roles
From the documentation
WHERE defines the MATCH patterns in more detail. The predicates are part of the
pattern description, not a filter applied after the matching is done.
This means that WHERE should always be put together with the MATCH clause it belongs to.
when you do start n=node(*) where n.name="xyz" you need to pass the result explicitly into your next optional matches. But when you do MATCH (n) WHERE n.name="xyz" this tells graph specifically what node to start looking into.
EDIT
Here is the thing. The documentation says Optional Match returns null if a pattern is not found so in your first case, it includes all those results too where n.username property is null or cases where n doesnt even have a relationship suggested in the OPTIONAL MATCH pattern. So when you do a WITH n , the graph is explicitly told to use only n.
Excerpt from the documentation (link : here)
OPTIONAL MATCH matches patterns against your graph database, just like MATCH does.
The difference is that if no matches are found, OPTIONAL MATCH will use NULLs for
missing parts of the pattern. OPTIONAL MATCH could be considered the Cypher
equivalent of the outer join in SQL.
Either the whole pattern is matched, or nothing is matched. Remember that
WHERE is part of the pattern description, and the predicates will be
considered while looking for matches, not after. This matters especially
in the case of multiple (OPTIONAL) MATCH clauses, where it is crucial to
put WHERE together with the MATCH it belongs to.
Also few more things to note about the behaviour of WHERE clause: here
Excerpts:
WHERE is not a clause in it’s own right — rather, it’s part of MATCH,
OPTIONAL MATCH, START and WITH.
In the case of WITH and START, WHERE simply filters the results.
For MATCH and OPTIONAL MATCH on the other hand, WHERE adds constraints
to the patterns described. It should not be seen as a filter after the
matching is finished.