Q.Explain the usage of HAVING clause in GROUP BY command in RDBMS with the help of an example.

The HAVING clause filters groups created by GROUP BY, just as WHERE filters individual rows before grouping. You cannot use WHERE to test aggregate conditions because aggregates are computed after WHERE runs; HAVING solves this by applying conditions after the grouping and aggregation are complete.

Consider a table Sales(Region, Product, Amount). If you want regions where total sales exceed 50000, you write:

SELECT Region, SUM(Amount) 
FROM Sales 
GROUP BY Region 
HAVING SUM(Amount) > 50000;

Here GROUP BY collects rows by Region, SUM computes the total for each group, and HAVING keeps only those groups meeting the condition. Without HAVING, you would retrieve all regional totals; with it, you see only high-performing regions. …

When you work with relational databases, you often need to summarize data — count how many customers placed orders, find the average salary by department, or total sales by region. The GROUP BY clause lets you collapse rows into groups and apply aggregate functions like COUNT, SUM, AVG, MAX, or MIN to each group. But what if you want to filter those groups themselves? That's where HAVING steps in.

The WHERE clause cannot do this job because it operates on individual rows before any grouping happens. It examines each record one by one and decides whether to include it in the result set. Once GROUP BY has done its work and created summary groups, WHERE is already finished. You need a tool that works after grouping, on the aggregated results themselves. HAVING is precisely that tool.

Think of the sequence this way: the database first applies WHERE to filter rows, then GROUP BY to organize the survivors into groups, then HAVING to filter those groups based on aggregate conditions, and finally SELECT to display the result. HAVING always appears after GROUP BY in the SQL statement and uses aggregate functions in its condition.

A Concrete Example

Suppose you manage a bookstore and have a table called Sales with columns: BookID, BookTitle, Quantity, and Price. You want to find which books have sold more than 50 copies in total.

SELECT BookTitle, SUM(Quantity) AS TotalSold
FROM Sales
GROUP BY BookTitle
HAVING SUM(Quantity) > 50;

Here's what happens step by step. The GROUP BY clause groups all sales records by BookTitle, so all rows for "The Great Gatsby" collapse into one group, all rows for "1984" into another, and so on. For each group, SUM(Quantity) calculates the total copies sold. Now HAVING examines each group's total: if it exceeds 50, that group appears in the final result; otherwise, it's excluded.

Consider another scenario: you have an Employees table with columns EmployeeID, Department, and Salary. You want departments where the average salary exceeds 60,000.

SELECT Department, AVG(Salary) AS AvgSalary
FROM Employees
GROUP BY Department
HAVING AVG(Salary) > 60000;
``` …