SQL Aggregate functions

SQL Aggregate Functions are used to perform calculations on a set of rows and return a single value. These functions are particularly useful when we need to summarize, analyze, or group large datasets in SQL databases. Whether you are working with sales data, employee records or product inventories, aggregate functions help us derive meaningful insights.

They are often used with the GROUP BY clause in SQL to summarize data for each group. Commonly used aggregate functions include COUNT(), SUM(), AVG(), MIN() and MAX(). In this article, we will explain the most commonly used SQL aggregate functions, their syntax, practical examples, and how to make the best use of them in your queries.

Key Features of SQL Aggregate Functions:

• Operate on groups of rows: They work on a set of rows and return a single value.
• Ignore NULLs: Most aggregate functions ignore NULL values, except for COUNT(*).
• Used with GROUP BY: To perform calculations on grouped data, you often use aggregate functions with GROUP BY.
• Can be combined with other SQL clauses: Aggregate functions can be used alongside HAVING, ORDER BY, and other SQL clauses to filter or sort results.

Commonly Used SQL Aggregate Functions

Below are the most frequently used aggregate functions in SQL.

1. Count()

The COUNT() function returns the number of rows that match a given condition or are present in a column.

• COUNT(*): Counts all rows.
• COUNT(column_name): Counts non-NULL values in the specified column.
• COUNT(DISTINCT column_name): Counts unique non-NULL values in the column.

Examples:

-- Total number of records in the table
SELECT COUNT(*) AS TotalRecords FROM Employee;

-- Count of non-NULL salaries
SELECT COUNT(Salary) AS NonNullSalaries FROM Employee;

-- Count of unique non-NULL salaries
SELECT COUNT(DISTINCT Salary) AS UniqueSalaries FROM Employee;

2. SUM()

The SUM() function calculates the total sum of a numeric column.

• SUM(column_name): Returns the total sum of all non-NULL values in a column.

Examples:

-- Calculate the total salary
SELECT SUM(Salary) AS TotalSalary FROM Employee;

-- Calculate the sum of unique salaries
SELECT SUM(DISTINCT Salary) AS DistinctSalarySum FROM Employee;

3. AVG()

The AVG() function calculates the average of a numeric column. It divides the sum of the column by the number of non-NULL rows.

• AVG(column_name): Returns the average of the non-NULL values in the column.

Examples:

-- Calculate the average salary
SELECT AVG(Salary) AS AverageSalary FROM Employee;

-- Average of distinct salaries
SELECT AVG(DISTINCT Salary) AS DistinctAvgSalary FROM Employee;

4. MIN() and MAX()

The MIN() and MAX() functions return the smallest and largest values, respectively, from a column.

• MIN(column_name): Returns the minimum value.
• MAX(column_name): Returns the maximum value.

Examples:

-- Find the highest salary
SELECT MAX(Salary) AS HighestSalary FROM Employee;

-- Find the lowest salary
SELECT MIN(Salary) AS LowestSalary FROM Employee;

Examples of SQL Aggregate Functions

Let's consider a demo Employee table to demonstrate SQL aggregate functions . This table contains employee details such as their ID, Name, and Salary.

1. Count the Total Number of Employees

SELECT COUNT(*) AS TotalEmployees FROM Employee;

Output:

2. Calculate the Total Salary

SELECT SUM(Salary) AS TotalSalary FROM Employee;

Output:

3. Find the Average Salary:

SELECT AVG(Salary) AS AverageSalary FROM Employee;

Output:

4. Find the Highest and Lowest Salary

SELECT MAX(Salary) AS HighestSalary FROM Employee;

Output:

Using Aggregate Functions with GROUP BY

SQL GROUP BY allows us to group rows that have the same values in specific columns. We can then apply aggregate functions to these groups, which helps us summarize data for each group. This is commonly used with the COUNT(), SUM(), AVG(), MIN(), and MAX() functions.

Example: Total Salary by Each Employee

SELECT Name, SUM(Salary) AS TotalSalary
FROM Employee
GROUP BY Name;

Output:

Using HAVING with Aggregate Functions

The HAVING clause is used to filter results after applying aggregate functions, unlike WHERE, which filters rows before aggregation. HAVING is essential when we want to filter based on the result of an aggregate function.

Example: Find Employees with Salary Greater Than 600

SELECT Name, SUM(Salary) AS TotalSalary
FROM Employee
GROUP BY Name
HAVING SUM(Salary) > 600;

Output:

Key Takeaways about SQL Aggregate Functions

• Aggregate functions in SQL operate on a group of values and return a single result.
• They are often used with the GROUP BY clause to summarize the grouped data.
• Aggregate function operates on non-NULL values only (except COUNT).
• Commonly used aggregate functions are - MIN(), MAX(), COUNT(), AVG(), and SUM().

Conclusion

SQL Aggregate Functions are fundamental tools for summarizing large datasets. Whether you are calculating totals, averages, or finding maximum and minimum values, aggregate functions enable you to quickly perform complex calculations on your data. By mastering these functions, you can gain valuable insights, spot trends, and perform deep analysis on any dataset. With practice, you can leverage these functions with GROUP BY, HAVING, and other clauses to create powerful and efficient SQL queries.

kartik

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Article Tags :

• DBMS
• SQL
• Databases