https://docs.tigergraph.com/gsql-ref/current/tutorials/gsql-101/
Every graph has a schema, or a model describing the types of vertices and edges that appear in a graph.（描述图中有哪些类型的点边，每种类型有哪些属性，以及点边类型之间有哪些关系）
a graph showing a group of friends and when each of them met
- schema：
the Friendship edge answers the question, “What type of entity is connected by friendship?” - graph：
