Neo4j create constraint - neo4j tutorial - graph database

How to Create a Constraint using Cypher?

A constraint allows you to place restrictions over the data that can be entered against a node or a relationship.
Constraints help enforce data integrity, because they prevent users from entering the wrong kind of data.

Neo4j constraints

If a someone tries to enter the wrong kind of data when a constraint has been applied, they will receive an error message.

In Neo4j, you can create uniqueness constraints and property existence constraints.
Uniqueness Constraint
Specifies that the property must contain a unique value (i.e. no two nodes with an Artist label can share a value for the Nameproperty.)
Property Existence Constraint
Ensures that a property exists for all nodes with a specific label or for all relationships with a specific type. Property existence constraints are only available in the Neo4j Enterprise Edition.

To create a uniqueness constraint in Neo4j, use the CREATE CONSTRAINT ON statement.

CREATE CONSTRAINT ON (a:Artist) ASSERT a.Name IS UNIQUE

In the above example, we create a uniqueness constraint on the Name property of all nodes with the Artist label.
When the statement succeeds,the following message is displayed:

Note:When you create a constraint, Neo4j will create an index. Cypher will use that index for lookups just like other indexes.Therefore, there's no need to create a separate index. In fact, if you try to create a constraint when there's already an index, you will get an error.

:schema

You will see the newly created constraint, as well as the index that was created with it. We can also see the index that was created previously:

You can test that the constraint actually works by attempting to create the same artist twice.
Run the following statement twice:

CREATE (a:Artist {Name: "Joe Satriani"}) 
RETURN a

Property existence constraints can be used to ensure all nodes with a certain label have a certain property.
For example, you could specify that all nodes labelled with Artist must contain a Name property.
To create a property existence constraint, use the ASSERT exists(variable.propertyName) syntax.

CREATE CONSTRAINT ON (a.Artist) ASSERT exists(a.Name)