How to get only unique data from the database?

Question

PHP

Tell me the format of the query to the database
there is a table (for example):

.     id    |  data_id  
-------------------------
      1     |     1
-------------------------
      2     |     1
-------------------------
      3     |     2
-------------------------
      4     |     1
-------------------------
      5     |     3

you need to get an array of MySQL / PostgreSQL databases at the output - it doesn’t matter .. Everything would be fine, but I don’t understand what question to ask the great Google)))
data_id = array(1,2,1,3);

Answer 1

select data_id from table where id <> data_id

Answer 2

select distinct data_id from tablename;