GROUP BY, HAVING, and Aggregations in SQL Server Explained

Part 9: GROUP BY, HAVING, and Aggregations in SQL Server

Microsoft SQL Server Tutorial Series: Beginner to Expert

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Welcome to Part 9 of our SQL Server tutorial series! In this lesson, we’ll explore how to summarize and group data using SQL Server’s powerful aggregation functions, GROUP BY and HAVING clauses.

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🎯 What You'll Learn

• How to group data using GROUP BY
• How to use aggregate functions like COUNT, SUM, AVG, MIN, MAX
• How to filter grouped results with HAVING
• Practical examples with real-world use cases
• Best practices for performance and clarity

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🧠 What is GROUP BY?

The GROUP BY clause groups rows that have the same values in specified columns into summary rows. It's typically used with aggregate functions.

SELECT Column1, AGG_FUNCTION(Column2)
FROM TableName
GROUP BY Column1;

Example: Count students per active status:

SELECT IsActive, COUNT(*) AS TotalStudents
FROM Students
GROUP BY IsActive;

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📊 Common Aggregate Functions in SQL Server

Function	Description	Example
COUNT()	Counts number of rows	COUNT(*)
SUM()	Total of a numeric column	SUM(Marks)
AVG()	Average value	AVG(Fees)
MIN()	Lowest value	MIN(Age)
MAX()	Highest value	MAX(Score)

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📌 Using HAVING to Filter Groups

HAVING is like WHERE, but for grouped results.

SELECT CourseID, COUNT(*) AS Enrolled
FROM Enrollments
GROUP BY CourseID
HAVING COUNT(*) > 10;

This returns only courses with more than 10 enrollments.

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📚 Real-World Example: Students per Year

SELECT YEAR(BirthDate) AS BirthYear, COUNT(*) AS StudentCount
FROM Students
GROUP BY YEAR(BirthDate)
ORDER BY BirthYear;

This query groups students by their year of birth and counts how many students were born each year.

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💡 Best Practices

Practice	Why It Matters
Use aliases for aggregated columns	Improves readability (e.g., COUNT(*) AS Total)
Use WHERE before GROUP BY	Filter raw data before grouping for performance
Use HAVING for filtering groups only	HAVING is evaluated after GROUP BY
Format numbers and dates in SELECT	Use FORMAT() if needed for presentation

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🧾 Quick SQL Cheat Sheet

-- Count rows per group
SELECT Department, COUNT(*) AS Total
FROM Employees
GROUP BY Department;

-- Filter groups
SELECT Category, SUM(Price) AS TotalSales
FROM Products
GROUP BY Category
HAVING SUM(Price) > 10000;
  

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✅ Summary

• Use GROUP BY to group data by columns
• Apply aggregate functions to summarize data
• Use HAVING to filter aggregated results
• Combine WHERE, GROUP BY, and HAVING for full power

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🔗 What’s Next?

• Part 8: SQL Joins (INNER, LEFT, RIGHT, FULL)
• Up Next → Part 10: Subqueries and Common Table Expressions (CTEs)

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