SQL Query to get Distinct Values of all column values in a table

Question

How to retrieve distinct values from all columns in a Table.

Select Distinct col1, col2, col3, col4 from table1 will give the unique records by checking the equality of all the columns. But I need all disctinct values from a table? Thanks in advance.

For Ex: My table has below records:

Col1                  Col2                  Col3                    Col4
------------------------------------------------------------------------ 
Melbourne             Sit1                  10.0                    5.3 
Melbourne             Sit2                  10.1                    5.4 
Sydney                Sit1                  10.0                    5.3 
Sydney                Sit3                  10.0                    5.3 
Bangalore             Sit1                  10.5                    0.1

The end result should be like below:

Result
----------------
Melbourne
Sydney
Bangalore
Sit1
Sit2
Sit3
10.0
10.1
10.5
5.3
5.4
5.1

Order doesn't matter.

Answer 1

You can do it like this:

select col1 from table union
select col2 from table union
. . .
select coln from table;

The use of union removes duplicate values. Note that this assumes that the column types are compatible (such as all being strings).

EDIT:

If the column in a table has to have only one type. (Okay, it could be a variant, but that is probably not a simplification.) You can cast everything to varchar2() :

select cast(col1 as varchar2(255)) from table union
select cast(col2 as varchar2(255)) from table union
. . .
select cast(coln as varchar2(255)) from table;

Alternatively, you could group the columns by data type and do a separate run for each one, or a separate column for each data type. It is unlikely that you will have exact matches across data types, so this might meet your need.