4. Differences between conceptual schema, user view, and internal schema:
Conceptual Schema:
- The conceptual schema represents the overall logical view of the entire database system.
- It defines the entities, their attributes, and the relationships among the entities.
- It provides a high-level abstraction and hides the technical details of the database management system (DBMS) from the users and developers.
- It serves as a blueprint for designing the user views and internal schema of the database system.
User View:
- User views are specific subsets of the conceptual schema that are designed to meet the information requirements of individual users or user groups.
- User views are tailored to the specific needs of users by providing a simplified and customized perspective of the database system.
- They specify the particular data items and operations that users can access and manipulate.
- User views can be implemented using the underlying conceptual schema or by additional restrictions and transformations.
Internal Schema:
- The internal schema represents the physical view of the database system.
- It defines how the data is stored and organized in the underlying storage structures, such as files, indexes, and data blocks.
- It includes details regarding the data representation, storage allocation, indexing techniques, and access paths used by the DBMS.
- The internal schema is optimized for efficient storage, retrieval, and manipulation of data.
- It is typically transparent to the users and developers, as they interact with the database system through the conceptual schema and user views.
In summary, the conceptual schema defines the overall logical view of the database system, the user views provide customized perspectives for individual users, and the internal schema represents the physical storage and organization of data.
5. A company database needs to store information about employees (identified by ssn, with salary and phone as attributes), departments (identified by dno, with dname and budget as attributes), and children of employees (with name and age as attributes). Employees work in departments; each department is managed by an employee; a child must be identified uniquely by name when the parent (who is an employee; assume that only one parent works for the company) is known. We are not interested in information about a child once the parent leaves the company. Draw an ER diagram that captures this information.
Here is the ER diagram that captures the information:
```
Employee
+---------------+
+----------------+ | ssn (PK) |
| ssn (PK) |<------| salary |
| salary | | phone |
| phone | +---------------+
+----------------+
Department
+----------------+
+----------------+ | dno (PK) |
| dno (PK) |<------| dname |
| dname | | budget |
| budget | +----------------+
| manager_ssn |
+----------------+
Child
+------------------+
+----------+ | name (PK) |
| name (PK) |<----------| age |
| age | | parent_ssn (FK) |
+----------+ +------------------+
```
Explanation:
- The entities in the diagram are Employee, Department, and Child.
- Employee has attributes ssn (primary key), salary, and phone.
- Department has attributes dno (primary key), dname, budget, and manager_ssn (foreign key referencing Employee's ssn).
- Child has attributes name (primary key), age, and parent_ssn (foreign key referencing Employee's ssn).
- The relationships are as follows:
- An Employee works in a Department (many-to-one relationship).
- A Department is managed by an Employee (one-to-one relationship).
- An Employee can have multiple Children (one-to-many relationship).
Please note that the diagram is a simplified representation of the entities and their relationships and may not capture all the complexities of the real-world scenario.
The conceptual schema, user view, and internal schema are all different aspects of a database system. Here are their differences:
1. Conceptual Schema:
- The conceptual schema represents the overall logical structure of the entire database.
- It defines the entities (objects) in the database, their attributes (properties), and the relationships between them.
- It is designed to be independent of any specific database management system (DBMS).
- It provides a high-level view of the database, focusing on the logical organization and relationships of the data.
2. User View:
- User views are specific representations of the database tailored for particular users or user groups.
- They are created by selecting and organizing the data from the conceptual schema to meet specific requirements or needs.
- User views simplify the complexity of the entire database by providing users with a customized, simplified, and limited perspective of the data.
- Each user view can present a different subset of data, customized attributes, or specific data transformations.
3. Internal Schema:
- The internal schema (also known as physical schema) describes the physical storage structure and organization of the database.
- It specifies how the data is stored in the database files.
- It defines the data representation format, the storage organization, and the access methods used to manipulate the data at the low level.
- The internal schema is typically designed by database administrators and is not directly exposed to users.
In summary, the conceptual schema represents the logical structure of the entire database, the user view is a customized representation of the data for specific users or groups, and the internal schema defines the physical storage structure and organization of the data in the database files.