A Neo4j best practice is to use an ID as a unique property value for each node.
Unique IDs help ensure duplicate data is not created. When you load data from CSV files, you rely heavily upon the IDs specified in the file. If the IDs in your CSV file are not unique for the same entity (node), you could create duplicate data. You may also have problems loading the data and creating relationships between nodes.
You can add constraints to your database to stop the creation of nodes with duplicate IDs.
Create a unique constraint
The syntax for creating a unique constraint on a property is:
CREATE CONSTRAINT [constraint_name] [IF NOT EXISTS]
FOR (n:LabelName)
REQUIRE n.propertyName IS UNIQUEThe constraint is for a property on all nodes with a specified label.
The constraint_name is optional, but it is good practice to specify one. If you do not specify a constraint name, Neo4j will create one for you.
The IF NOT EXISTS clause is also optional - if not used Neo4j will generate an error if the constraint already exists.
Person node constraint
The Person nodes you created should all have a unique tmbdId property.
You can create a constraint for the tmdbId property to ensure that all Person nodes have a unique tmdbId property value.
Review the following Cypher statement.
CREATE CONSTRAINT Person_tmdbId IF NOT EXISTS
FOR (x:Person)
REQUIRE x.tmdbId IS UNIQUEYou should note that:
-
The constraint name is
Person_tmdbId. -
The optional clause
IF NOT EXISTSis used - without which Neo4j would raise an error if the constraint exists. -
It applies to all nodes with the
Personlabel. -
It requires the
tmdbIdproperty to be unique.
Run the Cypher statement to create the constraint:
CREATE CONSTRAINT Person_tmdbId
FOR (x:Person)
REQUIRE x.tmdbId IS UNIQUEYou can check that the constraint has been created by running the following Cypher statement:
SHOW CONSTRAINTSYou should see the constraint named Person_tmdbId in the results.
If you try to create a Person node with a duplicate tmdbId property value, Neo4j will raise an error.
CREATE (p:Person {tmdbId: 3}) RETURN pNode(0) already exists with label `Person` and property `tmdbId` = 3Drop a constraint
If you need to drop a constraint, use the following Cypher statement.
DROP CONSTRAINT [constraint_name]Check Your Understanding
Duplicate Constraint
True or False - Neo4j will allow you to create duplicate unique ID constraints.
-
❏ True
-
✓ False
Hint
Using the 'IF NOT EXISTS' clause will prevent Neo4j from generating an error if the constraint already exists.
Solution
The statement is False. Neo4j will generate an error if a constraint exists - unless IF NOT EXISTS has been specified.
Summary
In this lesson, you learned how to create a unique constraint on a property.
In the next lesson, you will load a CSV file of movies and create Movie nodes.