open:cypher-query [EL]

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# Cypher Query

{{ https://i.imgur.com/sffPY48.jpg }}

## Case sensitivity

Case sensitive
  - Node labels (:Person)
  - Relationship types (:ACTED_IN)
  - Property keys (name)

Case insensitive
  - Cypher keywords (MATCH, return)

## Aggregates

Aggregate queries in Cypher are a little bit different than in SQL as we don't need to specify a grouping key.

We implicitly group by any non aggregate fields in the RETURN statement.

<code>
// implicitly groups by p.name
MATCH (p:Person)-[:ACTED_IN]->(m:Movie)
RETURN p.name, count(*) AS numberOfMovies
</code>

### Other aggregate functions

https://neo4j.com/docs/cypher-refcard/current/

## Constraints and Indexes

### Unique Constraints

We create unique constraints to:
  - ensure uniqueness
  - allow fast lookup of nodes which match label-property pairs.

<code>
CREATE CONSTRAINT ON (label:Label)
ASSERT label.property IS UNIQUE
</code>

There are three types of unique constraints:
  - Unique node property constraint
  - Node property existence constraint
  - Relationship property existence constraint

<code>
CREATE CONSTRAINT ON (label:Lable)
ASSERT EXISTS(label.name)
</code>

<code>
CREATE CONSTRAINT ON ()-[rel:REL_TYPE]->()
ASSERT EXISTS(rel.name)
</code>

### Indexes

We create indexes to:
  - allow fast lookup of nodes which match label-property pairs.

<code>
CREATE INDEX ON :Label(property)
</code>

The following predicates use indexes:
  - Equality
  - STARTS WITH
  - CONTAINS
  - ENDS WITH
  - Range searches
  - (Non-) existence checks

### The MERGE Clause

<code>
MERGE (p:Person {name:'Tom Hanks', oscar: true})
RETURN p
</code>

There is not a :Person node with name:'Tom Hanks' and oscar:true in the graph, but there is a :Person with name:'Tom Hanks'.

What do you think will happen here?

<code>
MERGE (p:Person {name:'Tom Hanks'})
SET p.oscar = true
RETURN p
</code>

## Write queries

### The CREATE Clause

<code>
CREATE (m:Movie {title:'Mystic River', released:2003})
RETURN m
</code>

### The SET Clause

<code>
MATCH (m:Movie {title:'Mystic River'})
SET m.tagline = 'We bury our sins here, Dave. We wash them clean.'
RETURN m
</code>

### The CREATE Clause

<code cypher>
MATCH (m:Movie {title:'Mystic River'})
MATCH (p:Person {name:'Kevin Bacon'})
CREATE (p)-[r:ACTED_IN {role:['Sean']}]->(m)
RETRUN p,r,m
</code>


### ON CREATE and ON MATCH

<code>
MERGE (p:Person {name: 'Your Name'})
ON CREATE SET p.created = timestamp(), p.updated= 0
On MATCH SET p.updated = p.updated + 1
RETURN p.created, p.updated
</code>

### Reading data from CSV with Cypher

<code>
[USING PREIODIC COMMIT] // optional transaction batching
LOAD CSV // load csv data
WITH HEADERS // optionally use first header row as keys in 'row' map
FROM "url" // file:// URL relative to $NEO4J_HOME/import or http://
AS row // return each row of the CSV as list of strings or map
FIELDTERMINATOR ";" // alternative field delimiter

... rest of the Cypher statement ...
</code>


### Tabular CSV records to Graph structure

Type the following command into the query pane in the browser:

<code>
:play http://guides.neo4j.com/fundamentals/import.html
</code>

## Developing Applications with Neo4j and Cypher

### Available APIs

  - Remote with Drivers
    - [[Bolt]] Binary Protocol
    - Officially Supported Drivers
    - Cypher transactional HTTP Endpoint
    - neo4j.com/developer/language-guides
  - Native Java API
    - User Defined Procedures
    - Execute Cypher
    - Core Java API