Presentation on Group By and Having Clause in SQL

Objective

This presentation focuses on two important SQL keywords, Group By and Having, explaining how they can be used to manage and analyze data effectively.

Group By Clause

Key Points:

• Purpose: To organize data into groups and perform aggregate functions on these groups.
• Example Scenario: If maintaining a student table, use Group By to find out how many students are in each class.
• Operation: Group By arranges data into groups based on shared values, allowing aggregation without individual conditions.

Example:

• Table: Employee (Columns: ID, Name, Department, Salary, City)
• SQL Query to count employees per city:

  SELECT COUNT(ID), City 
  FROM Employee 
  GROUP BY City;
  

• Output Observations: Aggregates data by city, showing New Delhi with 1 entry, Bangalore with 3, etc.

Having Clause

Key Points:

• Purpose: To filter the results of a Group By operation using conditions.
• Difference from WHERE: WHERE filters rows before grouping, while HAVING filters groups after they have been formed.

Example:

• Using the previous query to filter cities with more than two employees:

  SELECT COUNT(ID), City 
  FROM Employee 
  GROUP BY City 
  HAVING COUNT(ID) >= 2;
  

• Output: Displays only Bangalore (3 employees) and Chennai (2 employees).

Aggregate Functions

• Functions commonly used with Group By and Having include COUNT, SUM, MIN, MAX, and AVERAGE.

Homework Questions

1. Find the department-wise count of all employees.
2. Determine how many departments have more than two employees.

Conclusion

• Summary: Group By organizes data into sets of rows, and Having refines the result set based on conditions.
• Usage: Both clauses are fundamental for database analysis when dealing with aggregate data.

Thank you for your attention and engagement! Feel free to try the homework questions and share your solutions.

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