What is an aggregate function, and what are some common examples?

An aggregate function in SQL is a function that performs a calculation on a set of values and returns a single value. Aggregate functions are often used with the GROUP BY clause of the SELECT statement to summarize data by group.

Some common examples of aggregate functions in SQL include:

COUNT(): Returns the number of rows in a table or column.
SUM(): Returns the sum of all the values in a column.
AVG(): Returns the average of all the values in a column.
MIN(): Returns the minimum value in a column.
MAX(): Returns the maximum value in a column.
Here is an example of an aggregate function used with the GROUP BY clause:

SQL
SELECT country, COUNT(*) AS num_customers
FROM customers
GROUP BY country;
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This query will return the number of customers in each country.

Aggregate functions are a powerful tool for summarizing data in SQL. By understanding how to use aggregate functions, you can write more efficient and informative queries.

Here are some additional examples of how to use aggregate functions in SQL:

Calculate the total sales for each product:
SQL
SELECT product_name, SUM(sales) AS total_sales
FROM orders
GROUP BY product_name;
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Calculate the average age of employees in each department:
SQL
SELECT department_name, AVG(age) AS average_age
FROM employees
GROUP BY department_name;
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Find the employee with the highest salary:
SQL
SELECT name, salary
FROM employees
ORDER BY salary DESC
LIMIT 1;
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Find the number of employees who have worked at the company for more than 5 years:
SQL
SELECT COUNT(*) AS num_employees
FROM employees
WHERE years_employed > 5;
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