how to find unique element in sql

1DISTINCT
2- select distinct * from employees; ==> 
3           retrieves any row if it has at 
4           least a single unique column.
5- select distinct first_name from employees; ==> 
6              retrieves unique names
7              from table. (removes duplicates)
8- select distinct count(*) from employees;
9          retrieve number of unique rows
10          if any row has at least a single unique data.