GROUP BY « Select Query « MySQL Tutorial

2.3.1.'GROUP BY' directive can display the data grouped by field
2.3.2.You can use COUNT() if you want to find out how many employees each city has
2.3.3.With no GROUP columns is illegal if there is no GROUP BY clause
2.3.4.Group by NULL value
2.3.5.Use count(), OR and Group by
2.3.6.Count the number of employees per city only for employees whose description is known