How to use distinct in database

For example:

      There are have tabel_1 in the database :

 

      C1    C2

      1       2

      1       2

      1       3

 

 

Execute sql cmd:

               Select distinct C1 from table_1

 

result: C1

           1

 

 

Execute sql cmd:

               Select distinct  C1,C2 from table_1

 

result:  C1 C2

            1    2

            1    3

 

How the database execute this sql cmd? We can understand below:

     Frist, Select C1 from table_1   ***no distinct

     Second, Campare the Data, if same, delete,only exist diffience record.