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[cypher/en] proofread
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Boris Verkhovskiy
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097cd60e49
@ -5,84 +5,57 @@ contributors:
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- ["Théo Gauchoux", "https://github.com/TheoGauchoux"]
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---
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Cypher is the Neo4j’s query language to manipulate graphs easily. It reuses syntax from SQL and mixes it with kind of ascii-art to represent graphs.
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Cypher is Neo4j's query language for easily manipulating graphs.
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It reuses syntax from SQL and mixes it with kind of an ASCII-art to represent graphs.
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This tutorial assumes that you already know graph concepts like nodes and relationships.
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[Read more here.](https://neo4j.com/developer/cypher-query-language/)
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## Nodes represent a record in a graph
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`()` is an empty *node*, to indicate that there is a *node*, but it's not relevant for the query.
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Nodes
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---
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`(n)` is a *node* referred by the variable `n`, reusable in the query. It begins with lowercase and uses camelCase.
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**Represents a record in a graph.**
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`(p:Person)` - you can add a *label* to your node, here `Person`. It's like a type/class/category. It begins with uppercase and uses camelCase.
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`()`
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It's an empty *node*, to indicate that there is a *node*, but it's not relevant for the query.
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`(p:Person:Manager)` - a node can have many *labels*.
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`(n)`
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It's a *node* referred by the variable **n**, reusable in the query. It begins with lowercase and uses camelCase.
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`(p:Person {name : 'Théo Gauchoux', age : 22})` - a node can have some *properties*, here `name` and `age`. It begins with lowercase and uses camelCase.
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`(p:Person)`
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You can add a *label* to your node, here **Person**. It's like a type / a class / a category. It begins with uppercase and uses camelCase.
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The types allowed in properties:
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`(p:Person:Manager)`
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A node can have many *labels*.
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- Numeric
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- Boolean
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- String
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- List of previous primitive types
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`(p:Person {name : 'Théo Gauchoux', age : 22})`
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A node can have some *properties*, here **name** and **age**. It begins with lowercase and uses camelCase.
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*Warning: there's no datetime properties in Cypher! You can use a String with a specific pattern or a Numeric from a specific date.*
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The types allowed in properties :
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`p.name` - you can access a property with the dot style.
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- Numeric
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- Boolean
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- String
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- List of previous primitive types
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## Relationships (or Edges) connect two nodes
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*Warning : there isn't datetime property in Cypher ! You can use String with a specific pattern or a Numeric from a specific date.*
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`[:KNOWS]` is a *relationship* with the *label* `KNOWS`. It's a *label* as the node's label. It begins with uppercase and use UPPER\_SNAKE\_CASE.
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`p.name`
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You can access to a property with the dot style.
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`[k:KNOWS]` - the same *relationship*, referred by the variable `k`, reusable in the query, but it's not necessary.
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`[k:KNOWS {since:2017}]` - the same *relationship*, with *properties* (like *node*), here `since`.
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Relationships (or Edges)
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---
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`[k:KNOWS*..4]` is structural information to use in a *path* (seen later). Here, `\*..4` says "Match the pattern, with the relationship `k` which can be repeated between 1 and 4 times.
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**Connects two nodes**
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## Paths - the way to mix nodes and relationships.
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`[:KNOWS]`
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It's a *relationship* with the *label* **KNOWS**. It's a *label* as the node's label. It begins with uppercase and use UPPER\_SNAKE\_CASE.
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`(a:Person)-[:KNOWS]-(b:Person)` - a path describing that `a` and `b` know each other.
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`[k:KNOWS]`
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The same *relationship*, referred by the variable **k**, reusable in the query, but it's not necessary.
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`(a:Person)-[:MANAGES]->(b:Person)` - a path can be directed. This path describes that `a` is the manager of `b`.
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`[k:KNOWS {since:2017}]`
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The same *relationship*, with *properties* (like *node*), here **since**.
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`(a:Person)-[:KNOWS]-(b:Person)-[:KNOWS]-(c:Person)` - you can chain multiple relationships. This path describes the friend of a friend.
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`[k:KNOWS*..4]`
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It's a structural information to use in a *path* (seen later). Here, **\*..4** says "Match the pattern, with the relationship **k** which be repeated between 1 and 4 times.
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`(a:Person)-[:MANAGES]->(b:Person)-[:MANAGES]->(c:Person)` - a chain can also be directed. This path describes that `a` is the boss of `b` and the big boss of `c`.
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Commonly used patterns (from Neo4j documentation):
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Paths
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---
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**The way to mix nodes and relationships.**
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`(a:Person)-[:KNOWS]-(b:Person)`
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A path describing that **a** and **b** know each other.
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`(a:Person)-[:MANAGES]->(b:Person)`
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A path can be directed. This path describes that **a** is the manager of **b**.
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`(a:Person)-[:KNOWS]-(b:Person)-[:KNOWS]-(c:Person)`
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You can chain multiple relationships. This path describes the friend of a friend.
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`(a:Person)-[:MANAGES]->(b:Person)-[:MANAGES]->(c:Person)`
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A chain can also be directed. This path describes that **a** is the boss of **b** and the big boss of **c**.
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Patterns often used (from Neo4j doc) :
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```
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// Friend-of-a-friend
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```cypher
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// Friend-of-a-friend
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(user)-[:KNOWS]-(friend)-[:KNOWS]-(foaf)
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// Shortest path
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@ -91,17 +64,15 @@ path = shortestPath( (user)-[:KNOWS*..5]-(other) )
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// Collaborative filtering
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(user)-[:PURCHASED]->(product)<-[:PURCHASED]-()-[:PURCHASED]->(otherProduct)
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// Tree navigation
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// Tree navigation
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(root)<-[:PARENT*]-(leaf:Category)-[:ITEM]->(data:Product)
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```
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Create queries
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---
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## Create queries
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Create a new node
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```
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```cypher
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CREATE (a:Person {name:"Théo Gauchoux"})
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RETURN a
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```
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@ -110,52 +81,51 @@ RETURN a
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Create a new relationship (with 2 new nodes)
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```
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```cypher
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CREATE (a:Person)-[k:KNOWS]-(b:Person)
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RETURN a,k,b
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```
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Match queries
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---
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## Match queries
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Match all nodes
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```
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```cypher
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MATCH (n)
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RETURN n
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```
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Match nodes by label
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```
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```cypher
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MATCH (a:Person)
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RETURN a
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```
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Match nodes by label and property
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```
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```cypher
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MATCH (a:Person {name:"Théo Gauchoux"})
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RETURN a
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```
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Match nodes according to relationships (undirected)
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```
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```cypher
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MATCH (a)-[:KNOWS]-(b)
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RETURN a,b
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```
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Match nodes according to relationships (directed)
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```
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```cypher
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MATCH (a)-[:MANAGES]->(b)
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RETURN a,b
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```
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Match nodes with a `WHERE` clause
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```
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```cypher
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MATCH (p:Person {name:"Théo Gauchoux"})-[s:LIVES_IN]->(city:City)
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WHERE s.since = 2015
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RETURN p,state
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@ -163,19 +133,17 @@ RETURN p,state
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You can use `MATCH WHERE` clause with `CREATE` clause
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```
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```cypher
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MATCH (a), (b)
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WHERE a.name = "Jacquie" AND b.name = "Michel"
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CREATE (a)-[:KNOWS]-(b)
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```
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Update queries
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---
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## Update queries
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Update a specific property of a node
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```
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```cypher
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MATCH (p:Person)
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WHERE p.name = "Théo Gauchoux"
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SET p.age = 23
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@ -183,7 +151,7 @@ SET p.age = 23
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Replace all properties of a node
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```
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```cypher
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MATCH (p:Person)
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WHERE p.name = "Théo Gauchoux"
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SET p = {name: "Michel", age: 23}
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@ -191,27 +159,25 @@ SET p = {name: "Michel", age: 23}
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Add new property to a node
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```
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```cypher
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MATCH (p:Person)
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WHERE p.name = "Théo Gauchoux"
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SET p + = {studies: "IT Engineering"}
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SET p += {studies: "IT Engineering"}
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```
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Add a label to a node
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```
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```cypher
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MATCH (p:Person)
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WHERE p.name = "Théo Gauchoux"
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SET p:Internship
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```
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Delete queries
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---
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## Delete queries
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Delete a specific node (linked relationships must be deleted before)
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```
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```cypher
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MATCH (p:Person)-[relationship]-()
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WHERE p.name = "Théo Gauchoux"
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DELETE relationship, p
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@ -219,17 +185,17 @@ DELETE relationship, p
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Remove a property in a specific node
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```
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```cypher
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MATCH (p:Person)
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WHERE p.name = "Théo Gauchoux"
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REMOVE p.age
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```
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*Pay attention to the `REMOVE`keyword, it's not `DELETE` !*
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*Pay attention to the `REMOVE` keyword, it's not `DELETE`!*
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Remove a label from a specific node
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```
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```cypher
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MATCH (p:Person)
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WHERE p.name = "Théo Gauchoux"
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DELETE p:Person
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@ -237,32 +203,28 @@ DELETE p:Person
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Delete entire database
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```
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```cypher
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MATCH (n)
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OPTIONAL MATCH (n)-[r]-()
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DELETE n, r
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```
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*Seriously, it's the `rm -rf /` of Cypher !*
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*Seriously, it's the `rm -rf /` of Cypher!*
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## Other useful clauses
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Other useful clauses
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---
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`PROFILE` - before a query, show its execution plan.
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`PROFILE`
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Before a query, show the execution plan of it.
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`COUNT(e)` - count entities (nodes or relationships) matching `e`.
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`COUNT(e)`
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Count entities (nodes or relationships) matching **e**.
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`LIMIT x` - limit the result to the first `x` results.
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`LIMIT x`
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Limit the result to the x first results.
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## Special hints
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Special hints
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---
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- There is just single-line comments in Cypher, with double-slash : // Comments
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- You can execute a Cypher script stored in a **.cql** file directly in Neo4j (it's an import). However, you can't have multiple statements in this file (separated by **;**).
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- Cypher only has single-line comments, using double-slashes: `// comment`
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- You can execute a Cypher script stored in a .cql file directly in Neo4j (it's an import). However, you can't have multiple statements in this file (separated by `;`).
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- Use the Neo4j shell to write Cypher, it's really awesome.
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- The Cypher will be the standard query language for all graph databases (known as **OpenCypher**).
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- Cypher will be the standard query language for all graph databases (known as [openCypher](https://opencypher.org/)).
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Read more [here](https://neo4j.com/developer/cypher-query-language/).
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