sql unique constraint on a 2 columns combination

Question

How can you create a unique constraint on a combination of two values in two columns.

meaning

column1  column2 
   2        1 

looking for the constraint to disallow

column1  column2 
   1        2 

Answer 1

If your database allows expressions in an index you can do something like this (ANSI SQL):

CREATE UNIQUE INDEX on your_table (least(column1, column2)
                              , greatest(column1, column2));

Note this is a unique index not a unique constraint. The only difference for most DBMS is that you can't have a unique index as the target of a foreign key, but otherwise they serve the same purpose.

If your DBMS does not have least() or greatest() you can replace that using a CASE expression:

create unique index on your_table 
  (case column1 < column2 then column1 else column2 end, 
   case column2 > column1 then column2 else column1 end));