TW Neo4j

From the documentation NULLs in Neo4j are explained as. null is used to represent missing/undefined values. null is not equal to null . Not knowing two values does not imply that they are the same value. So the expression null = null yields null and not true . To check if an expression is null , use IS NULL . Arithmetic expressions, comparisons and function calls (except coalesce ) will return null if any argument is null . An attempt to access a missing element in a list or a property that doesn’t exist yields null . In OPTIONAL MATCH clauses, null s will be used for missing parts of the pattern. All very neat and dry and abstract. NULLS are undefined, they have no value that can be determined. In Neo4j, only non-null properties are stored. A different concept to the traditional database way of looking at the world. Each Node, of the same label, can have entirely different properties, and will have null return values where no determinable value for a property has been...

WITH allows you to pass on data from one part of the query to the next. Whatever you list in WITH will be available in the next query part. You can use aggregation, SKIP, LIMIT, ORDER BY with WITH much like in RETURN. The only difference is that your expressions have to get an alias with AS alias to be able to access them in later query parts. That means you can chain query parts where one computes some data and the next query part can use that computed data. In your case it is what GROUP BY and HAVING would be in SQL but WITH is much more powerful than that. here is another example match (n:Employee)-[r1:WORKSIN]->(a:Team) with distinct a order by a.name limit 10 match (a)-[:INBUILDING]->(c:Property) return c.name WITH is just like RETURN just within a query. it can select, aggregate, limit, sort it's arguments. The only difference is that you need to name each expression. Any after the WITH statement only the values / variables that are passed along are accessible/visible. ...

There may be a time where many user sessions attempt to update the same property on the same node. Unlikely, but possible. This may be a 'sequence' node, where you're keeping track of an increasing identifier, a sort of autoincrement. How can you be, almost, certain that your CYPHER statement will not be blocked by another session updating the node property at that exact time? Assume the sequence node: CREATE (s:Sequences {counterToIncrement: 0}) Then call the following CYPHER MATCH (s:Sequences) CALL apoc.atomic.add(s,'counterToIncrement',1,10) YIELD newValue as seq RETURN seq This will attempt to add 1 to the sequence property 'counterToIncrement' , and will attempt up to 10 times if the property is locked, and finally the new value of 'counterToIncrement' is yielded and returned. From a post : https://community.neo4j.com/t5/neo4j-graph-platform/integer-sequence-generator-in-neo4j-apoc/td-p/19403  

A Node is the 'Table' of the Graph Database world.   The main entity in a graph. They are also sometimes referred to as vertices or points. In our ASCII art world, a pair of parentheses is used to represent nodes. This resembles a circle and provides a simplified syntax for representing a node in ASCII:     ()     (node) The variable (node) holds node values so they can be processed or returned in a query later on. If you do not need to do anything with the node, you can skip the use of the variable. 

Delete nodes without labels MATCH (n) WHERE size(labels(n)) = 0 DETACH DELETE n Similarly you can find nodes without labels using: MATCH (n) WHERE size(labels(n)) = 0 RETURN n

From Neo4j v3.5 and onwards you can use a generic CYPHER statement: match (s)-[r]->(e) with s,e,type(r) as typ, tail(collect(r)) as coll foreach(x in coll | delete x) to remove all duplicate relationships in your Neo4j Graph Database. You can refine the control over what is deleted by adding in a node label or relationship type. We're collecting the relationships and grouping by type (as well as start and end node), so if any collection is greater than 1, then there are multiple relationships with the same type. TAIL removes the first element from the collect statement, but the first relationship back into rows then delete them. If you don't have labels to use for your query, then the query will be graph-wide, which is likely to take much longer to execute, and may encounter issues if the set of relationships to delete is too large to handle all at once.

To show a visualisation of the Nodes and Relationships in your Neo4j instance, run the following in your Neo4j Browser: CALL db.schema.visualisation()

 How many Nodes and relationships are in your Graph? You could look on the left hand side of the Neo4j Browser, but that would be cheating. For nodes: MATCH (n) RETURN count(n) For Relationships: MATCH ()-[r]->() RETURN count(r) The above CYPHER statements assume all nodes and relationships are to be counted. Once you start adding in labels and relationships you will filter the return values For Outgoing Relationships: MATCH (a)-[r]->() RETURN count(a) This counts nodes with outgoing relationships only.

Recipe for creating new ‘blank’ database. Putty onto the server  cd /etc/neo4j sudo vi neo4j.template Uncomment and edit and change one line – the name of the database #***************************************************************** # Neo4j Community configuration # # For more details and a complete list of settings, please see # https://neo4j.com/docs/operations-manual/current/reference/configuration-settings/ #***************************************************************** # The name of the database to mount dbms.active_database=prod.db cypher.forbid_shortestpath_common_nodes=false # Paths of directories in the installation. dbms.directories.data=/var/lib/neo4j/data restart the machine / service. When you now check the dbms.directories.data directory there is another directory alongside the default graph.db The browser now shows an empty database with no nodes, relationships or properties.

Node Properties in Neo4j. A property can be of any datatype even within nodes of the same label. Nodes have no fixed schema as with the RDBMS where most notions of data storage are formed and 'best practice' is enforced.  Properties themselves do not have types, but the values they hold do. However, you can have a property, X which could be assigned a string value or a numeric value. There is no enforcement of typing for a property. Of course, it would not (always) make sense to assign a property different type values in your graph. So, just what datatype is the property? Trusty APOC has a function for that. WITH [true, 42, 'foo', 1.2] AS data UNWIND data as value RETURN apoc.meta.type(value) So, just what datatype is the property? "BOOLEAN" "INTEGER" "STRING" "FLOAT" You can use this function in MATCHES on nodes in your system where properties are unknown at runtime. MATCH (n:MyNode) RETURN apoc.meta.type(n.MyProperty)

Deleting of Nodes:  To delete a node it should have no relationships Delete all nodes and relationships match (n)-[r]-() delete n, r Delete all nodes which have no relationships match (n) delete n Delete only ExampleNodes which have no relationships match (n:ExampleNode) delete n Delete all relationships match (n)-[r]-() delete r Delete only ExampleRelation relationships match (n)-[r:ExampleRelation]-() delete r

Controlling the service System services are controlled with the systemctl command. It accepts a number of commands and follows the format: systemctl {start|stop|restart} neo4j you may find that you need to prefix the systemctl command with sudo like: sudo systemctl stop neo4j looking at the output of the status command we get :   sudo systemctl status neo4j ● neo4j.service - Neo4j Graph Database    Loaded: loaded (/etc/systemd/system/neo4j.service; enabled; vendor preset: enabled)    Active: active (running) since Thu 2020-07-23 09:53:29 UTC; 1h 33min ago  Main PID: 958 (pre-neo4j.sh)     Tasks: 64    Memory: 14.3G       CPU: 22min 1.740s    CGroup: /system.slice/neo4j.service            ├─ 958 /bin/bash /etc/neo4j/pre-neo4j.sh            ├─ 996 /bin/sh /etc/neo4j/reset-password-aws.sh            ├─ 997 /usr/bin/java -cp /va...

In a Graph Database Relationships between the nodes are considered to be as important, if not more important than the nodes themselves. A Relationship joins two nodes or 'loops-back' to the same node. The relationship can be created or deleted but cannot be renamed. How to delete relationship MATCH (n { name: 'Greg' })-[r:KNOWS]->() DELETE r To rename a Relationship you do a simultaneous create and delete. Create a new relationship and delete the old as below: Rename relationship MATCH (n)-[rel:IS_CONNECTED_TO]->(m) MERGE (n)-[:KNOWS]->(m) DELETE rel MATCH (n)-[rel:IS_CONNECTED_TO]->(m) WITH rel CALL apoc.refactor.setType(rel, 'KNOWS') YIELD input, output RETURN *

Labels are the 'TABLES' of the Graph database world. Here's some useful methods of manipulating Labels. Rather than delete and create your Nodes in Neo4j just to change the label, it may be easier to try one of the two methods below: Rename node - Method 1: MATCH (s:OldLabel) SET s:NewLabel REMOVE s:OldLabel Rename node - Method 2: MATCH (n:OldLabel) WITH COLLECT(n) AS nodes CALL apoc.refactor.rename.label('OldLabel','NewLabel'[,nodes]) YIELD errorMessages AS eMessages RETURN eMessages; Do you have any more ways of 'renaming' Nodes in Neo4j? Nodes can also have more than one Label. Here is how to add and remove a Label on a Node where the only matching property is {name : 'Greg'} How to add a new label MATCH (n { name: 'Greg' }) SET n:NewLabel RETURN n.name, labels(n) AS labels How to remove a label MATCH (n { name: 'Greg' }) REMOVE n:OldLabel RETURN n.name, labels(n) AS labels

Neo4j is a graph database. It is not like Oracle or SQL Server - no tables and primary keys with associated foreign keys. Sounds .......... scary Sounds........... different But when you map out a workflow on a piece of paper, drawing boxes and circles and joinging them with lines, you create a graph. It's what comes naturally.