How Logical schema and physical schema are related?
I will explain this concepts with the help of this picture bellow:
We will take as an example an Oracle database(but this apply's to most of RDBMS).
- is made out of actually what you plan(your initial data model) and it's objects are:
- pkgs etc...
-here is where you have your data files, redo logs, control files etc, physical objects that reside on a O.S. and are in row state. These objects are used by logical structures to make our data make sense and readable.
How these two schema exist in the Database?
A schema is the set of metadata (data dictionary) used by the database, typically generated using DDL. It defines attributes of the database, such as tables, columns, and properties. A database schema is a description of the data in a database.
One or more schemas can reside on the same database.
How can these be manipulated according to need?
A schema is a collection of database objects. A schema is owned by a database user and has the same name as that user. Schema objects are logical structures created by users. Objects may define areas of the database to hold data, such as tables or indexes, or may consist just of a definition, such as a views or synonyms.
Objects in the same schema can use storage in different tablespaces, and a tablespace can hold contain storage from different schemas.
Schema objects can be created and manipulated using SQL. As an administrator, you can create and manipulate schema objects, just as you can the logical and physical structures of your database.
i hope it my answer helped.