CREATE UNIQUE

Match meets Create

The CREATE UNIQUE clause is a mix of MATCH and CREATE — it will match what it can, and create what is missing.

Reference: CREATE UNIQUE manual page
Related: :help MATCH :help CREATE :help MERGE :help Cypher
MATCH (us:Country)
WHERE us.name = 'United States'
CREATE UNIQUE (us)-[:STATE]->(ca {name:'California'})
RETURN ca
Finds the country named "United States". If found, will then create a new state named California, but only if it doesn't already exist.