What is database schema?
Database schema is a visual representation of database. The structure is described by formal language and is supported by database management system (DBMS). It represents logical view of the database.
The database schema has a list of attributes and instructions for database engine showing how data is organized and how the components are associated in a database.
A database schema describes entities and relationship among them.
What is schema?
In its literal meaning, the term schema is called “form” or “shape” of the database which shows that how the data is organized in database.
A schema is a representation of design or idea in the shape of model. A database may contain one or multiple schemas.
A database schema consists of objects like tables, views and stored procedure etc. In other words, we can say that database schema is a container of objects.
A collection of database objects is called schema in Oracle Database. These objects are associated with a database user name who is called schema owner.
Depending upon database privileges, user can have complete control over these objects.
What is database instance?
Database schema is the skeleton of a database. Database schema is designed before creation of database. A database schema does not contain any information or data.
While a database instance is a state of an operational database having information or data. It tends to change with the time. A database management system(DBMS) ensures that its every instance follows validation, constraints, and conditions imposed by database designers and that every instance is in a valid state.
The working is performed by programmers and database administrators at this level. The data records are stored is stored in data structures. The implementation of data structure is not visible at this level. Design of database at logical level is called logical schema.
It describes logical constraints that apply to the stored data. It may define integrity constraints, views, and tables.
Database design at view level is called view schema. It pertains to interaction of end user with database systems.
The schema instructs about how information stored, therefore, if any change made in schema is replicated among all the domain controllers.