Difference between merge and create unique in Neo4j

31
votes

I'm trying to figure out what is the difference between MERGE and CREATE UNIQUE. I know these features:

MERGE

I'm able to create node, if doesn't exist pattern. 

    MERGE (n { name:"X" }) RETURN n;

This create node "n" with property name, empty node "m" and relationship RELATED.

    MERGE (n { name:"X" })-[:RELATED]->(m) RETURN n, m;

CREATE UNIQUE

I'm not able to create node like this.

    CREATE UNIQUE (n { name:"X" }) RETURN n;

If exists node "n", create unique makes empty node "m" and relationship RELATED.

    MATCH (n { name: 'X' }) CREATE UNIQUE (n)-[:RELATED]->(m) RETURN n, m;

If this pattern exists, nothing created, only returns pattern.

From my point of view, I see MERGE and CREATE UNIQUE are quite same queries, but with CREATE UNIQUE you can't create start node in relationship. I would be grateful, if someone could explain this issue and compare these queries, thx.

1
mergeneo4jcypher

1 Answers

38
votes

CREATE UNIQUE has slightly more obscure semantics than MERGE. MERGE was developed as an alternative with more intuitive behavior than CREATE UNIQUE; if in doubt, MERGE is usually the right choice.

The easiest way to think of MERGE is as a MATCH-or-create. That is, if something in the database would MATCH the pattern you are using in MERGE, then MERGE will just return that pattern. If nothing matches, the MERGE will create all missing elements in the pattern, where a missing element means any unbound identifier.

Given

MATCH (a {uid:123})
MERGE (a)-[r:LIKES]->(b)-[:LIKES]->(c)

"a" is a bound identifier from the perspective of the MERGE. This means cypher somehow already knows which node it represents.

This statement can have two outcomes. Either the whole pattern already exists, and nothing will be created, or parts of the pattern are missing, and a whole new set of relationships and nodes matching the pattern will be created.

Examples

// Before merge:
(a)-[:LIKES]->()-[:LIKES]->()

// After merge:
(a)-[:LIKES]->()-[:LIKES]->()


// Before merge:
(a)-[:LIKES]->()-[:OWNS]->()

// After merge:
(a)-[:LIKES]->()-[:OWNS]->()
(a)-[:LIKES]->()-[:LIKES]->()


// Before merge:
(a)

// After merge:
(a)-[:LIKES]->()-[:LIKES]->()