how to select unique element in sql

1SELECT DISTINCT column1, column2, ...
2
3 FROM table_name; 

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

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.

1This constraint ensures all values in a column are unique.
2Example 1 (MySQL): Adds a unique constraint to the id column when
3creating a new users table.
4CREATE TABLE users (
5id int NOT NULL,
6name varchar(255) NOT NULL,
7UNIQUE (id)
8);
9Example 2 (MySQL): Alters an existing column to add a UNIQUE
10constraint.
11ALTER TABLE users
12ADD UNIQUE (id);