Oracle: How to avoid duplicate rows in query result while selecting from table

If you select data from one column in a table like this:

select columnName
  from tableName;


columnName
=========
A
A
A
B
B

you would get that columns data from all rows. If you would like to avoid / skip duplicates (and only get the distinct values) you could type:

select distinct columnName

  from tableName;

columnName

=========

A

B

Another approach is:

select columnName, count(*)

  from tableName

  group by columnName;

columnName   count(*)

=========    ========

A            3

B            2