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diff --git a/cypher.html.markdown b/cypher.html.markdown new file mode 100644 index 00000000..6efc0cec --- /dev/null +++ b/cypher.html.markdown @@ -0,0 +1,228 @@ +--- +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**). + |