Difference between the HAVING and WHERE statements

 Answer:

The main difference between WHERE and HAVING in SQL Server is:

1. WHERE

   • Filters rows before grouping happens.

   • Can be used with any column in the table (raw data).

   • Cannot be used with aggregate functions like COUNT(), SUM(), etc.

2. HAVING

   • Filters rows after grouping has been done using GROUP BY.

   • Is mainly used with aggregate functions to filter grouped results.

   • Can also be used without GROUP BY (acts like a WHERE in that case).

 Example:

-- WHERE filters before grouping
SELECT Department, COUNT(*) AS EmployeeCount
FROM Employees
WHERE Department <> 'HR'  -- filter before grouping
GROUP BY Department;

-- HAVING filters after grouping
SELECT Department, COUNT(*) AS EmployeeCount
FROM Employees
GROUP BY Department
HAVING COUNT(*) > 5;  -- filter after grouping

Execution Order in SQL Server:

1. FROM

2. WHERE (filters raw rows)

3. GROUP BY (groups remaining rows)

4. HAVING (filters grouped data)

5. SELECT

6. ORDER BY

 Summary Table:

Feature	WHERE	HAVING
Filters Before Grouping	Yes	No
Filters After Grouping	No	Yes
Works with Aggregates	No	Yes
Requires GROUP BY	No	No (optional)

 In short:

• Use WHERE to filter rows before grouping.

• Use HAVING to filter grouped results, especially when using aggregate functions.

Using WHERE  -> Filter rows before aggregation

SELECT Product, SUM(Quantity) AS TotalQty FROM Sales WHERE Quantity > 10 GROUP BY Product

This only groups rows where Quantity > 10 (ignores rows ≤ 10).

Using HAVING -> Filter after aggregation

SELECT Product, SUM(Quantity) AS TotalQty FROM Sales GROUP BY Product HAVING SUM(Quantity) > 20

This groups all rows first  then shows only products where the total quantity is more than 20.