Question

I have a table which contains data as

col1  col2  
a      b  
b      a  
c      d
d      c
a      d 
a      c

For me row 1 and row 2 are duplicate because a, b & b, a are the same. The same stands for row 3 and row 4.

I need an SQL (not PL/SQL) query which gives output as

col1   col2
a       b
c       d
a       d
a       c

Answer 1

select distinct least(col1, col2), greatest(col1, col2)
from your_table

Edit: for those using a DBMS that does support the standard SQL functions least and greatest this can be simulated using a CASE expression:

select distinct 
       case 
         when col1 < col2 then col1
         else col2
       end as least_col, 
       case 
         when col1 > col2 then col1
         else col2
        end as greatest_col 
from your_table