- It is a physical representation of data that appears in the database management system.
- In simple words, a schema is the structure of any database.
- It defines how the data is stored in a database and relationship among those data, but it does not show the data available in those tables.
- The database schema includes the definition of the database, record types (entities), and the components that make up those records.
- Schema of a database does not change by performing certain operations like insertion, updation, and deletion, but it can only be modified or changed by modifying the DDL statements.
- Database schema describes the integrity constraints of the database, domains of all attributes, foreign, and primary key of all the relations.
- Schema is of three types:
- Physical Schema: The overall design of a database at the physical level is called a physical schema. This schema describes how the data is stored in the secondary storage devices.
- Logical Schema: The overall design of a database at the logical level is called a logical schema. A database administrator (DBA) and the programmers used to work at this level.
- View Schema: The overall design of a database at a view level is called view schema. This schema generally describes the user’s interaction with the database system.
Example of Schema:
- A subschema is the sub-level or subset of the schema having the same features that a schema has.
- This type of schema allows a user to view only that part of the database in which he is interested.
- A language called the SDL (Subschema Definition Language) is used to specify a subschema in a database management system (DBMS).
- For example: Suppose an employee table in a database where the programmer can access all fields of this table, but the user can access only two or three fields of it. Subschema describes both views of the database.
- The data or information stored in the database at a particular moment is called an instance of the database.
- An instance can also be called as a current state or database state.
- The database schema defines the variables in tables that belong to a specific database; the values or records of these variables at a particular moment are called the instance of the database.
- Many instances can be constructed to correspond to a specific database schema.
- Every time we insert, modify, or delete the value of data item in a record, one state of data changes into another state.