DATABASE MANAGEMENT SYSTEM

Query By Example (QBE) and Domain Relational Calculus are two different approaches to expressing queries in a Database Management System (DBMS). Let's explore each of them:

Query By Example (QBE):

1. Purpose:

• QBE is a visual and intuitive approach to query databases.
• It allows users to express queries by example, using a graphical interface rather than writing SQL commands.

2. Components:

• Tables and Forms: Users interact with tables and forms to create queries.
• Query Grid: Users specify conditions and criteria by filling out a grid-like structure.
• Example:

| ID | Name   | Age | Salary |
|----|--------|-----|--------|
|    | [John] |     |   >50k |

3. Example QBE Query:

• In the above grid example, the user is querying for records where the name is "John" and the salary is greater than 50,000.

In QBE we don’t write complete queries like SQL or other database languages it comes with some blank so we need to just fill that blanks and we will get our required result.

Example

Consider the example where a table college_dept present in the database with Name, Phone_Number and Branch fields. And we want to get the name of Course-Representative name who belongs to the MCA Branch.

If we write this query in SQL we have to write it like

SELECT NAME FROM COLLEGE_DEPT WHERE BRANCH = 'MCA'"

In the case of QBE, it may be done as like there is a field present and we just need to fill it with “MCA” and then click on SEARCH button we will get our required result.

4. Advantages:

• Intuitive for users who are not familiar with SQL.
• Provides a visual representation of the query.

5. Disadvantages:

• Limited expressiveness compared to SQL for complex queries.
• May not be as efficient for users proficient in SQL.

Domain Relational Calculus:

1. Purpose:

• Domain Relational Calculus is a formal language used to express queries in a logical and mathematical way.
• It defines what data is needed without specifying how to retrieve it.

2. Components:

• Variables: Represent elements in the database.
• Quantifiers (Existential and Universal): Specify the existence or universality of variables.
• Predicates: Define conditions.

3. Example Domain Relational Calculus Query:

This query selects all employees named John with a salary greater than 50,000.

4. Advantages:

• Provides a formal and precise way to express queries.
• Allows for more complex and expressive queries.

5. Disadvantages:

• Less intuitive for users not familiar with formal logic.
• Requires a deeper understanding of mathematical concepts.