Getting started with Neo4j - The Graph Database

Our system configuration is: Windows 10, 64-bit OS and 8 GB RAM

The first thing to do after installing Neo4j from this link: https://neo4j.com/download/

Next thing we do is create a graph in the "Neo4j Desktop" application.




We create a graph named "Graphene" and set it's password.




We start our graph "Graphene" by clicking on "Open Browser".

### We create a Node with a Label using the Cypher Query Language:

A label in Neo4j is used to group (classify) the nodes using labels. You can create a label for a node in Neo4j using the CREATE clause.

Syntax:

CREATE (node:label) 

Example:

CREATE (Dhawan:player) [ Ref: https://www.tutorialspoint.com/neo4j/neo4j_cql_creating_nodes.htm ]

OR

CREATE (a:Greeting)

### Next, we "Create Node with Properties":

Properties are the key-value pairs using which a node stores data. You can create a node with properties using the CREATE clause. You need to specify these properties separated by commas within the flower braces “{ }”.

Syntax:

CREATE (node:label { key1: value, key2: value, . . . . . . . . .  }) 

Example: 

CREATE (Dhawan:player{name: "Shikar Dhawan", YOB: 1985, POB: "Delhi"}) 

This operation can also be done using a differently written query:

CREATE (a:Greeting) SET a.message = "Hola!"

### Query to get the complete graph as output in the browser view:

MATCH (n) RETURN n

### Querying an already created node:

CREATE (a:Greeting)  
SET a.message = ‘Hola’

QUERY: MATCH (a:Greeting {message: "Hola"}) RETURN a

### Deleting all nodes:

In Neo4j, DELETE statement is always used with MATCH statement to delete whatever data is matched. The DELETE command is used in the same place we used the RETURN clause in our previous examples.

MATCH (n) DELETE n

Ref: https://www.javatpoint.com/neo4j-delete-a-node

### The following query will delete the node "Kohli" where name is "Virat Kohli" with all its relationship.
MATCH (Kohli) DETACH DELETE Kohli

Ref: https://www.javatpoint.com/neo4j-delete-a-node

### Create a Relationship between existing Nodes

MATCH statement is used to create relationship between the existing Nodes.

Syntax:

MATCH (a:LabeofNode1), (b:LabeofNode2)   
WHERE a.name = "nameofnode1" AND b.name = " nameofnode2"   
CREATE (a)-[: Relation]->(b)   
RETURN a,b  

### Match by Relationship
You can retrieve nodes based on relationship using the MATCH clause.

Syntax
Following is the syntax of retrieving nodes based on the relationship using the MATCH clause.

MATCH (node:label)<-[: Relationship]-(n) 
RETURN n 
Example
Following is a sample Cypher Query to retrieve nodes based on relationship using the MATCH clause.

MATCH (Ind:Country {name: "India", result: "Winners"})<-[: TOP_SCORER_OF]-(n) 
RETURN n.name 

Ref: https://www.tutorialspoint.com/neo4j/neo4j_match_clause.htm

### Find all the relations between two nodes in Neo4j

MATCH (x:Test)-[r:CONNECTED_TO*]->(z:Test)
RETURN x, r, z

Ref: https://stackoverflow.com/questions/27848429/can-i-find-all-the-relations-between-two-nodes-in-neo4j

### Code to create a node using the Python package 'neo4j'

from neo4j import GraphDatabase

class HelloWorldExample(object):

    def __init__(self, uri, user, password):
        self._driver = GraphDatabase.driver(uri, auth=(user, password))

    def close(self):
        self._driver.close()

    def print_greeting(self, message):
        with self._driver.session() as session:
            greeting = session.write_transaction(self._create_and_return_greeting, message)
            print(greeting)

    @staticmethod
    def _create_and_return_greeting(tx, message):
        result = tx.run("CREATE (a:Greeting) "
                        "SET a.message = $message "
                        "RETURN a.message + ', from node ' + id(a)", message=message)
        
        # To get no results for query:
        #result = tx.run("MATCH (a:Greeting {message: 'Holaa!'}) RETURN a")
        
        #print(result) # [neo4j.BoltStatementResult object at 0x0000020A6C2D87B8]

        rtnVal = result.single()
        if rtnVal is None:
            return False
        else:
            return rtnVal[0]

Testing the code:

obj = HelloWorldExample('bolt://localhost:7687', 'neo4j', 'graphene')

obj.print_greeting('Hola')
OUTPUT: Hola, from node 40

Ref: https://neo4j.com/developer/python/