Retrieve only distinct combinations - sql-server

Question

I'm having column as Node varchar(25) in MS-SQL Server.
The possible values are
node
-----
D-C
C-B
B-A
B-C
B-E
C-A
A-B
A-C
C-D
etc.
I want to retrieve the the distinct combinations from it.
E.g.:
node
----
D-C
C-B
B-A
B-E
C-A
Please tell the SQL for this.

Answer 1

You have two pieces of data pressed into one column. This is not ideal. So my solution first has to correct this:
SQL> create table mytable (node)
2 as
3 select 'D-C' from dual union all
4 select 'C-B' from dual union all
5 select 'B-A' from dual union all
6 select 'B-C' from dual union all
7 select 'B-E' from dual union all
8 select 'C-A' from dual union all
9 select 'A-B' from dual union all
10 select 'A-C' from dual union all
11 select 'C-D' from dual
12 /
Table created.
SQL> with structured_data as
2 ( select regexp_substr(node,'[^-]+',1,1) startnode
3 , regexp_substr(node,'[^-]+',1,2) endnode
4 from mytable
5 )
6 select distinct least(startnode,endnode) startnode
7 , greatest(startnode,endnode) endnode
8 from structured_data
9 /
STARTNODE ENDNODE
--------- -------
B E
A C
A B
C D
B C
5 rows selected.

Answer 2

select distinct(Node) from YOUR_TABLE;
Here is a SQLfiddle with examples : http://www.sqlfiddle.com/#!4/76484/2
I answered for oracle since the question is tagged oracle. For MS-Server, it's probably the same thing though...

Answer 3

Another approach. Very similar to what Rob van Wijk has posted in terms of using greatest() and least() functions, but without invocation of regular expression functions. We can calculate greatest and least of column value and its revers value - value returned by the reverse() function:
Note: reverse() function is undocumented function. Do not use it in the the production application(s).
with t1(col) as(
select 'D-C' from dual union all
select 'C-B' from dual union all
select 'B-A' from dual union all
select 'B-C' from dual union all
select 'B-E' from dual union all
select 'C-A' from dual union all
select 'A-B' from dual union all
select 'A-C' from dual union all
select 'D-C' from dual
)
select res_1 /* as well/or we can choose res_2 */
from (select distinct
greatest(col, reverse(col)) as res_1
, least(col, reverse(col)) as res_2
from t1)
Result:
RES_1
-----
D-C
B-A
C-A
C-B
E-B
Or #2
select col
from ( select col
, row_number() over(partition by greatest(col, reverse(col))
, least(col, reverse(col))
order by col) as rn
from t1
)
where rn = 1
Result:
COL
---
A-B
A-C
B-C
D-C
B-E