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Property Graph Model

Cypher is a graph query language which operates on a property graph. A property graph may be defined in graph theoretical terms as a directed, vertex-labeled, edge-labeled multigraph with self-edges, where edges have their own identity. In the property graph, we use the term node to denote a vertex, and relationship to denote an edge. See Wikipedia’s definitions for reference:

1. Definitions

In a property graph, the following elements may exist:

  • Entity

    • Node

    • Relationship

  • Path

  • Token

    • Label

    • Relationship type

    • Property key

  • Property

1.1. Entity

  • An entity has a unique, comparable identity which defines whether or not two entities are equal (see the Comparability CIP for more details).

  • An entity is assigned a set of properties, each of which are uniquely identified in the set by their respective property keys.

1.1.1. Node

  • A node is the basic entity of the graph, with the unique attribute of being able to exist in and of itself.

  • A node may be assigned a set of unique labels.

  • A node may have zero or more outgoing relationships.

  • A node may have zero or more incoming relationships.

1.1.2. Relationship

  • A relationship is an entity that encodes a directed connection between exactly two nodes, the source node and the target node.

  • An outgoing relationship is a directed relationship from the point of view of its source node.

  • An incoming relationship is a directed relationship from the point of view of its target node.

  • A relationship is assigned exactly one relationship type.

1.2. Path

  • A path represents a walk through a property graph and consists of a sequence of alternating nodes and relationships.

  • A path always starts and ends at a node.

  • The smallest possible path contains a single node, and is called an empty path.

  • A path has a length, which is an integer greater than or equal to zero, which is equal to the number of relationships in the path.

  • Equality of paths is detailed in the Comparability CIP.

1.3. Token

  • A token is a nonempty string of Unicode characters.

1.3.1. Label

  • A label is a token that is assigned to nodes only.

1.3.2. Relationship type

  • A relationship type is a token that is assigned to relationships only.

1.3.3. Property key

  • A property key is a token which uniquely identifies an entity’s property.

1.4. Property

  • A property is a pair consisting of a property key and a property value.

  • A property value is an instantiation of one of Cypher’s concrete, scalar types, or a list of a concrete, scalar type. See the type system CIP for reference.

2. Graph attributes

  • The size of the graph is an integer greater than or equal to zero, and is equal to the number of nodes in the graph.