The three schema of the database

Question

I have created a database in Access and right know i have to write a report. I know that the databasesystem has three forms of schemas: physical, conceptual and external.

Answer 1

The Three-level ANSI-SPARC Architecture aka three schema approach:

• An external schema is the database (with metadata including constraints) as seen by some user, a view of the conceptual schema.

• The conceptual schema is the database (with metatdata including constraints) per se, for an enterprise.

• The internal schema is the implementation.

Typically an ER diagram would not considered detailed enough to be an external schema or conceptual schema.

You may be confusing these with Conceptual, logical and physical data models in data modeling. (Those very wiki links are themselves confused about ANSI/SPARC.)

• A conceptual model is informal and/or incomplete. Eg an ER diagram.

• A logical model is formal and in some sense complete and implementation independent. Eg approximately an ANSI/SPARC external or conceptual schema.

• A physical model is implementation. Eg approximately an ANSI/SPARC internal schema.

However there is very little agreement about just how detailed and abstract logical and physical models are. Or for that matter conceptual models. Or how these models correspond to ANSI/SPARC schemas. Eg whether an arbitrary SQL index is internal/physical or can be part of a user's/DBA's external/internal/logical interface.

See What are Conceptual, Logical and Physical Data Models? and its link Conceptual, Logical, Physical: Clearing the Confusion.

From the point of view of the relational model and ANSI/SPARC, a logical model (external and conceptual schema/level) should completely describe tables and their columns, constraints and application meanings. But conceptual-logical-physical presentations generally reflect a poor understanding of the relational model.

So probably your ER model is a conceptual model among conceptual-logical-physical models. Although you could take it as a (partial) description of a logical model or external or conceptual schema. And maybe your assignment itself is confused.

You are going to have to refer to whatever definitions/references your client/instructor has given you for the assignment.