图数据库
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关于

图数据库是用来存储图结构的非关系型的结构化数据的，相比关系型数据库，在检索和图中游走等操作上有更好的优化。例如，查询朋友的朋友，关系数据库需要两次join，如果更多步查询，需要的join操作就会更多。

JanusGraph

gremlin> g.V().has('name', 'hercules').out('father').out('father').values('name')
==>saturn

g: for the current graph traversal.
V: for all vertices in the graph
has('name', 'hercules'): filters the vertices down to those with name property "hercules" (there is only one).
out('father'): traverse outgoing father edge’s from Hercules.
‘out('father')`: traverse outgoing father edge’s from Hercules’ father’s vertex (i.e. Jupiter).
name: get the name property of the "hercules" vertex’s grandfather.

Java client的例子

Traversal t = g.V().has("name", "pluto"); // Define a traversal
// Note the traversal is not executed/iterated yet
Vertex pluto = null;
if (t.hasNext()) { // Check if results are available
    pluto = g.V().has("name", "pluto").next(); // Get one result
    g.V(pluto).drop().iterate(); // Execute a traversal to drop pluto from graph
}
// Note the traversal can be cloned for reuse
Traversal tt = t.asAdmin().clone();
if (tt.hasNext()) {
    System.err.println("pluto was not dropped!");
}
List<Vertex> gods = g.V().hasLabel("god").toList(); // Find all the gods
created in 2020-02-10