learnxinyminutes-docs/cypher.md

name	filename	contributors
Cypher	LearnCypher.cql	Théo Gauchoux https://github.com/TheoGauchoux

Cypher is Neo4j's query language for easily manipulating graphs. It reuses syntax from SQL and mixes it with kind of an ASCII-art to represent graphs. This tutorial assumes that you already know graph concepts like nodes and relationships.

Nodes represent a record in a graph

() is an empty node, to indicate that there is a node, but it's not relevant for the query.

(n) is a node referred by the variable n, reusable in the query. It begins with lowercase and uses camelCase.

(p:Person) - you can add a label to your node, here Person. It's like a type/class/category. It begins with uppercase and uses camelCase.

(p:Person:Manager) - a node can have many labels.

(p:Person {name : 'Théo Gauchoux', age : 22}) - a node can have some properties, here name and age. It begins with lowercase and uses camelCase.

The types allowed in properties:

• Numeric
• Boolean
• String
• List of previous primitive types

Warning: there's no datetime properties in Cypher! You can use a String with a specific pattern or a Numeric from a specific date.

p.name - you can access a property with the dot style.

Relationships (or Edges) connect two nodes

[:KNOWS] is a relationship with the label KNOWS. It's a label as the node's label. It uses UPPER_SNAKE_CASE.

[k:KNOWS] - the same relationship, referred by the variable k, reusable in the query, but it's not necessary.

[k:KNOWS {since:2017}] - the same relationship, with properties (like node), here since.

[k:KNOWS*..4] is structural information to use in a path (seen later). Here, \*..4 says "Match the pattern, with the relationship k which can be repeated between 1 and 4 times.

Paths - the way to mix nodes and relationships.

(a:Person)-[:KNOWS]-(b:Person) - a path describing that a and b know each other.

(a:Person)-[:MANAGES]->(b:Person) - a path can be directed. This path describes that a is the manager of b.

(a:Person)-[:KNOWS]-(b:Person)-[:KNOWS]-(c:Person) - you can chain multiple relationships. This path describes the friend of a friend.

(a:Person)-[:MANAGES]->(b:Person)-[:MANAGES]->(c:Person) - a chain can also be directed. This path describes that a is the boss of b and the big boss of c.

Commonly used patterns (from Neo4j documentation):

// Friend-of-a-friend
(user)-[:KNOWS]-(friend)-[:KNOWS]-(foaf)

// Shortest path
path = shortestPath( (user)-[:KNOWS*..5]-(other) )

// Collaborative filtering
(user)-[:PURCHASED]->(product)<-[:PURCHASED]-()-[:PURCHASED]->(otherProduct)

// Tree navigation
(root)<-[:PARENT*]-(leaf:Category)-[:ITEM]->(data:Product)

Create queries

Create a new node

CREATE (a:Person {name:"Théo Gauchoux"})
RETURN a

RETURN allows to have a result after the query. It can be multiple, as RETURN a, b.

Create a new relationship (with 2 new nodes)

CREATE (a:Person)-[k:KNOWS]-(b:Person)
RETURN a,k,b

Match queries

Match all nodes

MATCH (n)
RETURN n

Match nodes by label

MATCH (a:Person)
RETURN a

Match nodes by label and property

MATCH (a:Person {name:"Théo Gauchoux"})
RETURN a

Match nodes according to relationships (undirected)

MATCH (a)-[:KNOWS]-(b)
RETURN a,b

Match nodes according to relationships (directed)

MATCH (a)-[:MANAGES]->(b)
RETURN a,b

Match nodes with a WHERE clause

MATCH (p:Person {name:"Théo Gauchoux"})-[s:LIVES_IN]->(city:City)
WHERE s.since = 2015
RETURN p,state

You can use MATCH WHERE clause with CREATE clause

MATCH (a), (b)
WHERE a.name = "Jacquie" AND b.name = "Michel"
CREATE (a)-[:KNOWS]-(b)

Update queries

Update a specific property of a node

MATCH (p:Person)
WHERE p.name = "Théo Gauchoux"
SET p.age = 23

Replace all properties of a node

MATCH (p:Person)
WHERE p.name = "Théo Gauchoux"
SET p = {name: "Michel", age: 23}

Add new property to a node

MATCH (p:Person)
WHERE p.name = "Théo Gauchoux"
SET p += {studies: "IT Engineering"}

Add a label to a node

MATCH (p:Person)
WHERE p.name = "Théo Gauchoux"
SET p:Internship

Delete queries

Delete a specific node (linked relationships must be deleted before)

MATCH (p:Person)-[relationship]-()
WHERE p.name = "Théo Gauchoux"
DELETE relationship, p

Remove a property in a specific node

MATCH (p:Person)
WHERE p.name = "Théo Gauchoux"
REMOVE p.age

Pay attention to the REMOVE keyword, it's not DELETE!

Remove a label from a specific node

MATCH (p:Person)
WHERE p.name = "Théo Gauchoux"
DELETE p:Person

Delete entire database

MATCH (n)
OPTIONAL MATCH (n)-[r]-()
DELETE n, r

Seriously, it's the rm -rf / of Cypher!

Other useful clauses

PROFILE - before a query, show its execution plan.

COUNT(e) - count entities (nodes or relationships) matching e.

LIMIT x - limit the result to the first x results.

Special hints

• Cypher only has single-line comments, using double-slashes: // comment
• You can execute a Cypher script stored in a .cql file directly in Neo4j (it's an import). However, you can't have multiple statements in this file (separated by ;).
• Use the Neo4j shell to write Cypher, it's really awesome.
• Cypher will be the standard query language for all graph databases (known as openCypher).

Read more here.