You can count the number of rows:
SELECT count(*) TotalRows FROM employees;
Or count the employees per department:
SELECT DepartmentId, count(*) NumEmployees FROM employees GROUP BY DepartmentId;
You can count over a column/expression with the effect that will not count the
SELECT count(ManagerId) mgr FROM EMPLOYEES;
(There is one null value managerID column)
You can also use DISTINCT inside of another function such as COUNT to only find the DISTINCT members of the set to perform the operation on.
SELECT COUNT(ContinentCode) AllCount , COUNT(DISTINCT ContinentCode) SingleCount FROM Countries;
Will return different values. The SingleCount will only Count individual Continents once, while the AllCount will include duplicates.
AllCount: 7 SingleCount: 5