52 lines
1.6 KiB
Markdown
52 lines
1.6 KiB
Markdown
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## Reading and Writing
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OpenCypher supports combining multiple reads and writes using the
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`WITH` clause. In addition to combining, the `MERGE` clause is provided which
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may create patterns if they do not exist.
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### WITH
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The write part of the query cannot be simply followed by another read part. In
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order to combine them, `WITH` clause must be used. The names this clause
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establishes are transferred from one part to another.
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For example, creating a node and finding all nodes with the same property.
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```opencypher
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CREATE (node {property: 42}) WITH node.property AS propValue
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MATCH (n {property: propValue}) RETURN n
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```
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Note that the `node` is not visible after `WITH`, since only `node.property`
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was carried over.
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This clause behaves very much like `RETURN`, so you should refer to features
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of `RETURN`.
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### MERGE
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The `MERGE` clause is used to ensure that a pattern you are looking for exists
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in the database. This means that if the pattern is not found, it will be
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created. In a way, this clause is like a combination of `MATCH` and `CREATE`.
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Example. Ensure that a person has at least one friend.
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```opencypher
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MATCH (n :Person) MERGE (n)-[:FriendOf]->(m)
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```
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The clause also provides additional features for updating the values depending
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on whether the pattern was created or matched. This is achieved with `ON
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CREATE` and `ON MATCH` sub clauses.
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Example. Set a different properties depending on what `MERGE` did.
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```opencypher
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MATCH (n :Person) MERGE (n)-[:FriendOf]->(m)
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ON CREATE SET m.prop = "created" ON MATCH SET m.prop = "existed"
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```
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For more details, click [this
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link](https://neo4j.com/docs/developer-manual/current/cypher/clauses/merge/).
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