Update 2.graph-modeling.md

docs-2.0/8.service-tuning/2.graph-modeling.md

@@ -68,6 +68,54 @@ nebula> MATCH (v1:player)-[e:temp]->() return collect(e.tmp);
 +----------------+
 ```
 
+### About dangling edges
+
+A dangling edge is an edge that only connects to a single vertex and only one part of the edge connects to the vertex.
+
+In Nebula Graph {{ nebula.release }}, dangling edges may appear in the following two cases.
+
+1. Insert edges with [INSERT EDGE](../3.ngql-guide/13.edge-statements/1.insert-edge.md) statement before the source vertex or the destination vertex exists.
+
+2. Delete vertices with [DELETE VERTEX](../3.ngql-guide/12.vertex-statements/4.delete-vertex.md) statement and the `WITH EDGE` option is not used. At this time, the system does not delete the related outgoing and incoming edges of the vertices. There will be dangling edges by default.
+
+Dangling edges may appear in Nebula Graph {{nebula.release}} as the design allow it to exist. And there is no MERGE statement like openCypher has. The existence of dangling edges depends entirely on the application level. You can use [GO](../3.ngql-guide/7.general-query-statements/3.go.md) and [LOOKUP](../3.ngql-guide/7.general-query-statements/5.lookup.md) statements to find a dangling edge, but cannot use the [MATCH](../3.ngql-guide/7.general-query-statements/2.match.md) statement to find a dangling edge.
+
+Examples:
+
+```bash
+// Insert an edge that connects two vertices which do not exist in the graph. The source vertex's ID is '11'. The destination vertex's ID is'13'. 
+
+nebula> CREATE EDGE IF NOT EXISTS e1 (name string, age int);
+nebula> INSERT EDGE e1 (name, age) VALUES "11"->"13":("n1", 1);
+
+// Query using the `GO` statement
+
+nebula> GO FROM "11" over e1 YIELD properties(edge);
++----------------------+
+| properties(EDGE)     |
++----------------------+
+| {age: 1, name: "n1"} |
++----------------------+
+
+// Query using the `LOOKUP` statement
+
+nebula> LOOKUP ON e1 YIELD EDGE AS r;
++-------------------------------------------------------+
+| r                                                     |
++-------------------------------------------------------+
+| [:e2 "11"->"13" @0 {age: 1, name: "n1"}]              |
++-------------------------------------------------------+
+
+// Query using the `MATCH` statement
+
+nebula> MATCH ()-[e:e1]->() RETURN e LIMIT 100;
++---+
+| e |
++---+
++---+
+Empty set (time spent 3153/3573 us)
+```
+
 ### Breadth-first traversal over depth-first traversal
 
 - Nebula Graph has lower performance for depth-first traversal based on the Graph topology, and better performance for breadth-first traversal and obtaining properties. For example, if model A contains properties "name", "age", and "eye color", it is recommended to create a tag `person` and add properties `name`, `age`, and `eye_color` to it. If you create a tag `eye_color` and an edge type `has`, and then create an edge to represent the eye color owned by the person, the traversal performance will not be high.