From efbcd3dcbbc34d552436a2cd8f4c647052c5a119 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Th=C3=A9o=20Gauchoux?= <theo.gauchoux.pro@gmail.com>
Date: Wed, 12 Apr 2017 18:30:09 +0200
Subject: [cypher/en] Write tiny tutorial on Cypher (#2703)

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+---
+language: cypher
+filename: LearnCypher.cql
+contributors:
+    - ["Théo Gauchoux", "https://github.com/TheoGauchoux"]
+---
+
+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.
+This tutorial assumes that you already know graph concepts like nodes and relationships.
+
+[Read more here.](https://neo4j.com/developer/cypher-query-language/)
+
+
+`// There is just single-line comments in Cypher`
+
+
+
+Nodes
+---
+
+**Represents a record in a graph.**
+
+**`()`** It's an empty *node*, to indicate that there is a *node*, but it's not relevant for the query.
+
+**`(n)`** It's a *node* refered by the variable **n**, reusable in the query. It begins with lowercase and uses camelCase.
+
+**`(p:Person)`** 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.
+
+**`(p:Person:Manager)`** A node can have many *labels*.
+
+**`(p:Person {name : 'Théo Gauchoux', age : 22})`** A node can have some *properties*, here **name** and **age**. It begins with lowercase and uses camelCase.
+
+The types allowed in properties :
+* Numeric
+* Boolean
+* String
+* List of previous primitive types
+
+**Warning : there isn't datetime property in Cypher ! You can use String with a specific pattern or a Numeric from a specific date.**
+
+**`p.name`** You can access to a property with the dot style.
+
+
+Relationships (or Edges)
+---
+
+**Connects two nodes**
+
+**`[:KNOWS]`** 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.
+
+**`[k:KNOWS]`** The same *relationship*, refered by the variable **k**, reusable in the query, but it's not necessary.
+
+**`[k:KNOWS {since:2017}]`** The same *relationship*, with *properties* (like *node*), here **since**.
+
+**`[k:KNOWS*..4]`** 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.
+
+
+Paths
+---
+
+**The way to mix nodes and relationships.**
+
+**`(a:Person)-[:KNOWS]-[b:Person]`** A path describing that **a** and **b** know each other.
+
+**`(a:Person)-[:MANAGES]->[b:Person]`** A path can be directed. This path describes that **a** is the manager of **b**.
+
+**`(a:Person)-[:KNOWS]-[b:Person]-[:KNOWS]-[c:Person]`** You can chain multiple relationships. This path describes the friend of a friend.
+
+**`(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**.
+
+Patterns often used (from Neo4j doc) :
+
+```
+// Friend-of-a-friend 
+(user)-[:KNOWS]-(friend)-[:KNOWS]-(foaf)
+
+// Shortest path
+path = shortestPath( (user)-[:KNOWS*..5]-(other) )
+
+// Collaborative filtering
+(user)-[:PURCHASED]->(product)<-[:PURCHASED]-()-[:PURCHASED]->(otherProduct)
+
+// Tree navigation 
+(root)<-[:PARENT*]-(leaf:Category)-[:ITEM]->(data:Product)
+
+```
+
+Create queries
+---
+
+Create a new node
+```
+CREATE (a:Person {name:"Théo Gauchoux"})
+RETURN a
+```
+*`RETURN` allows to have a result after the query. It can be multiple, as `RETURN a, b`.*
+
+Create a new relationship (with 2 new nodes)
+```
+CREATE (a:Person)-[k:KNOWS]-(b:Person)
+RETURN a,k,b
+```
+
+Match queries
+---
+
+Match all nodes
+```
+MATCH (n)
+RETURN n
+```
+
+Match nodes by label
+```
+MATCH (a:Person)
+RETURN a
+```
+
+Match nodes by label and property
+```
+MATCH (a:Person {name:"Théo Gauchoux"})
+RETURN a
+```
+
+Match nodes according to relationships (undirected)
+```
+MATCH (a)-[:KNOWS]-(b)
+RETURN a,b
+```
+
+Match nodes according to relationships (directed)
+```
+MATCH (a)-[:MANAGES]->(b)
+RETURN a,b
+```
+
+Match nodes with a `WHERE` clause
+```
+MATCH (p:Person {name:"Théo Gauchoux"})-[s:LIVES_IN]->(city:City)
+WHERE s.since = 2015
+RETURN p,state
+```
+
+You can use `MATCH WHERE` clause with `CREATE` clause
+```
+MATCH (a), (b)
+WHERE a.name = "Jacquie" AND b.name = "Michel"
+CREATE (a)-[:KNOWS]-(b)
+```
+
+Update queries
+---
+
+Update a specific property of a node
+```
+MATCH (p:Person)
+WHERE p.name = "Théo Gauchoux"
+SET p.age = 23
+```
+
+Replace all properties of a node
+```
+MATCH (p:Person)
+WHERE p.name = "Théo Gauchoux"
+SET p = {name: "Michel", age: 23}
+```
+
+Add new property to a node
+```
+MATCH (p:Person)
+WHERE p.name = "Théo Gauchoux"
+SET p + = {studies: "IT Engineering"}
+```
+
+Add a label to a node
+```
+MATCH (p:Person)
+WHERE p.name = "Théo Gauchoux"
+SET p:Internship
+```
+
+Delete queries
+---
+
+Delete a specific node (linked relationships must be deleted before)
+```
+MATCH (p:Person)-[relationship]-()
+WHERE p.name = "Théo Gauchoux"
+DELETE relationship, p
+```
+
+Remove a property in a specific node
+```
+MATCH (p:Person)
+WHERE p.name = "Théo Gauchoux"
+REMOVE p.age
+```
+*Pay attention to the `REMOVE`keyword, it's not `DELETE` !*
+
+Remove a label from a specific node
+```
+MATCH (p:Person)
+WHERE p.name = "Théo Gauchoux"
+DELETE p:Person
+```
+
+Delete entire database
+```
+MATCH (n)
+OPTIONAL MATCH (n)-[r]-()
+DELETE n, r
+```
+*Seriously, it's the `rm -rf /` of Cypher !*
+
+Other useful clauses
+---
+
+**`PROFILE`** Before a query, show the execution plan of it.
+**`COUNT(e)`** Count entities (nodes or relationships) matching **e**.
+**`LIMIT x`** Limit the result to the x first results.
+
+Special hints
+---
+
+* 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 (separed by **;**).
+* Use the Neo4j shell to write Cypher, it's really awesome.
+* The Cypher will be the standard query language for all graph databases (known as **OpenCypher**).
+
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