What are the three data models commonly used in databases?

① One and only one node has no parent node (this node is called root node).

② Other nodes except the root node have one and only one parent node.

The records in the hierarchical model can only be organized into a set of trees, not an arbitrary set of graphs. In the hierarchical model, the organization of records is no longer a chaotic map, but an "inverted" tree.

2. Grid model:

① Allow multiple nodes to have no parent node.

② A node can have multiple parent nodes.

The data in the grid model is represented by a collection of records, and the relationship between the data is represented by links (which can be regarded as pointers). The records in the database can be organized into a set of arbitrary charts.

3. Relationship model:?

A relational model uses a set of tables to represent data and relationships between data.

Each table has multiple columns, and each column has a unique column name.