As queries execute, they build up result rows. Cypher executes operations per-row.
When a query is made up of completely separate parts, unrelated to each other, and you don't want to split the single query into multiple queries, you sometimes need a way to reset cardinality back to 1 to avoid executing an operation multiple times.
For example, using the Movies graph, let's write a query to label actors, and then create a new :Movie node.
MATCH (p:Person)
WHERE (p)-[:ACTED_IN]->(:Movie)
SET p:Actor
CREATE (:Movie{title:'The Animatrix'})
When run all in a single query, you'll see that multiple 'The Animatrix' nodes were created, not just one. This is because the CREATE clause executes per row, and we have one row for each :Actor just before the CREATE executes.
While you could get around the extra node creations by using MERGE instead of CREATE, you still pay the cost of executing multiple redundant MERGE operations per row, db hits that are completely unnecessary.
To reset cardinality back to 1 before the next part of the query, you'll need to use WITH DISTINCT on some arbitrary value.
MATCH (p:Person)
WHERE (p)-[:ACTED_IN]->(:Movie)
SET p:Actor
WITH DISTINCT 1 AS ignored
CREATE (:Movie{title:'The Animatrix'})
