Databases Reference
In-Depth Information
• That specific properties on matched nodes and relationships must be present (or
absent), irrespective of their values.
• That certain properties on matched nodes and relationships must have specific
values.
• That other arbitrarily complex expression predicates must be met.
Compared to the
MATCH
clause, which describes structural relationships and assigns
identifiers to parts of the pattern,
WHERE
is more of a tuning exercise to filter down the
current pattern match. Let's imagine, for example, that we want to restrict the range of
plays in our results to those from Shakespeare's
final period
, which is generally accepted
to have begun around 1608. We do this by filtering on the
year
property of matched
WROTE_PLAY
relationships. To enable this filtering, we adjust the
MATCH
clause, binding
the
WROTE_PLAY
relationship to an identifier, which we'll call
w
. Relationship identifiers
come before the colon that prefixes a relationship's name. We then add a
WHERE
clause
that filters on this relationship's
year
property:
START
theater=
node
:venue(name=
'Theatre Royal'
),
newcastle=
node
:city(name=
'Newcastle'
),
bard=
node
:author(lastname=
'Shakespeare'
)
MATCH
(newcastle)<-[:STREET|CITY*1..2]-(theater)
<-[:VENUE]-()-[:PERFORMANCE_OF]->()-[:PRODUCTION_OF]->
(play)<-[w:WROTE_PLAY]-(bard)
WHERE
w.year > 1608
RETURN DISTINCT
play.title
AS
play
Adding this
WHERE
clause means that for each successful match, the Cypher execution
engine checks that the
WROTE_PLAY
relationship between the Shakespeare node and the
matched play has a
year
property with a value greater than 1608. Matches with a
WROTE_PLAY
relationship whose
year
value is greater than 1608 will pass the test; these
plays will then be included in the results. Matches that fail the test will not be included
in the results. By adding this clause, we ensure that only plays from Shakespeare's late
period are returned:
+---------------+
| play |
+---------------+
| "The Tempest" |
+---------------+
1 row
Processing Results
Cypher's
RETURN
clause allows us to perform some processing on the matched graph
data before returning it to the user (or the application) that executed the query.
