The GROUP BY clause in MySQL is used to group rows based on one or more columns. It is often used in combination with aggregate functions like SUM, COUNT, AVG etc, to perform calculations on groups of rows. Here's an example of using the GROUP BY clause:

Consider a table called orders with the following structure:

Syntax:

                
    CREATE TABLE orders (
      order_id INT PRIMARY KEY,
      customer_id INT,
      product VARCHAR(50),
      quantity INT,
      price DECIMAL(10,2)
    );
                
            

Suppose we want to calculate the total sales amount for each customer. We can use the GROUP BY clause along with the SUM function to achieve this:

Syntax:

                
    SELECT customer_id, SUM(quantity * price) AS total_sales FROM orders GROUP BY customer_id;
                
            

In the above example, the GROUP BY clause is applied to the customer_id column. This query will group the orders by customer and calculate the total sales amount for each customer using the SUM function.

The GROUP BY clause should be used when you want to perform aggregate calculations on groups of rows. It is typically used in scenarios where you need to summarize or analyze data based on specific columns. Common use cases include calculating totals, averages, counts, or identifying unique values within groups.

It's important to note that when using the GROUP BY clause, the SELECT statement can only include columns that are part of the GROUP BY clause or aggregate functions. Any other columns that are not part of the GROUP BY clause or aggregate functions will result in an error.

Remember to adjust the column names and table name in the example based on your specific scenario. When using the GROUP BY clause, it's recommended to also consider using other clauses like HAVING, ORDER BY, or WHERE to further refine the grouped results or apply additional filters.