How do different schema definition languages support three schema architecture?
Answers
In a three schema approach, most data-related description languages or tools associated with schemas focus on the "physical level" and "view level", with the "conceptual level" mostly used in combining the schema design itself. In relational databases, the physical model is explained using SQL DDL. Physical schemas in NoSQL can be fixed to particular records using JSON or XML. There are some languages to describe conceptual schemas. But now a day languages are not used for conceptual schemas but instead Entity relationship model, a diagramming and description tool is used. The "view level" or "external level" is implemented outside of data managers in user-side code. This is done using object relational mapping with the help of tools like Hibernate. These tools convert "physical layer" schemas structure to "external-friendly" structures. Here the user input UI elements is given to the application code. Code having "database layer" is implemented inside the API.