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Cypher Fundamentals

Reading Data from Neo4j

• Introduction to Cypher
• Retrieving Nodes
• Finding Relationships
• Traversing Relationships
• Finding Emil
• Filtering Queries
• Finding Specific Actors

Writing Data to Neo4j

• Creating Nodes
• Creating a Node
• Creating Relationships
• Creating a Relationship
• Updating Properties
• Adding Properties to a Movie
• Merge Processing
• Adding or Updating a Movie
• Deleting Data
• Deleting Emil

QUIZ

Creating Relationships

VideoTranscript  

Creating a relationship between two nodes

In this lesson you will learn how to write Cypher clauses to create relationships between existing nodes in the graph.

Just like you can use MERGE to create nodes in the graph, you use MERGE to create relationships between two nodes. First you must have references to the two nodes you will be creating the relationship for. When you create a relationship between two nodes, it must have:

• Type

• Direction

For example, if the Person and Movie nodes both already exist, we can find them using a MATCH clause before creating the relationship between them.

cypher   Copy to ClipboardRun in Sandbox

MATCH (p:Person {name: 'Michael Cain'})
MATCH (m:Movie {title: 'The Dark Knight'})
MERGE (p)-[:ACTED_IN]->(m)

Here we find the two nodes that we want to create the relationship between. Then we use the reference to the found nodes to create the ACTED_IN relationship.

We can confirm that this relationship exists as follows:

cypher   Copy to ClipboardRun in Sandbox

MATCH (p:Person {name: 'Michael Cain'})-[:ACTED_IN]-(m:Movie {title: 'The Dark Knight'})
RETURN p, m

By default, in Neo4j Browser, the visualization connects nodes that have relationships between them.

Notice also that you need not specify direction in the MATCH pattern since the query engine will look for all nodes that are connected, regardless of the direction of the relationship.

For example, if we specified this relationship pattern:

cypher   Copy to ClipboardRun in Sandbox

MATCH (p:Person {name: 'Michael Cain'})<-[:ACTED_IN]-(m:Movie {title: 'The Dark Knight'})
RETURN p, m

This query returns no nodes since there are no nodes with the ACTED_IN relationship to Person nodes in the graph.

Creating nodes and relationships using multiple clauses

We can also chain multiple MERGE clauses together within a single Cypher code block.

cypher   Copy to ClipboardRun in Sandbox

MERGE (p:Person {name: 'Chadwick Boseman'})
MERGE (m:Movie {title: 'Black Panther'})
MERGE (p)-[:ACTED_IN]-(m)

This code creates two nodes and a relationship between them. Because we have specified the variables p and m, we can use them in the code to create the relationship between the two nodes.

Note that in this MERGE clause where we create the relationships, we did not specify the direction of the relationship. By default, if you do not specify the direction when you create the relationship, it will always be assumed left-to-right.

We can confirm that this relationship exists as follows:

cypher   Copy to ClipboardRun in Sandbox

MATCH (p:Person {name: 'Chadwick Boseman'})-[:ACTED_IN]-(m:Movie {title: 'Black Panther'})
RETURN p, m

Using MERGE to create nodes and a relationship in single clause

What MERGE does is create the node or relationship if it does not exist in the graph.

This code successfully creates the nodes and relationship:

cypher   Copy to ClipboardRun in Sandbox

MERGE (p:Person {name: 'Emily Blunt'})-[:ACTED_IN]->(m:Movie {title: 'A Quiet Place'})
RETURN p, m

You can execute this Cypher code multiple times and it will not create any new nodes or relationships.

Check your understanding

1. Creating relationships

From what you have learned thus far, what clause do you use to create a relationship?

•  

  INSERT

•  

  ADD

•  

  ADD RELATIONSHIP

•  

  MERGE

2. Using MERGE for relationships

Suppose you want to create the LIKES relationship between a reference to a node a, and a reference to a node b where the direction of the relationship is from a to b.

Which statements below will create the LIKES relationship from a to b?

•  

  MERGE (a)-[LIKES]→(b)

•  

  MERGE (a)-[:LIKES]→(b)

•  

  MERGE (a)-[LIKES]-(b)

•  

  MERGE (a)-[:LIKES]-(b)

3. Create relationship between two existing nodes

Suppose our graph has a Person node for Lucille Ball and a Movie node for Mame. How do we create the ACTED_IN relationship between these two nodes?

Use the dropdown below complete the code.

Once you have selected your option, click the Check Results query button to continue.

cypher  

MERGE (p:Person {name: 'Lucille Ball'})

-[ACTED_IN]->
-[:ACTED_IN]->
<-[ACTED_IN]-
-[ACTED_IN]-


(m:Movie {title: 'Mame'})
RETURN p, m

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