Swap unique indexed column values in database

Question

I have a database table and one of the fields (not the primary key) is having a unique index on it. Now I want to swap values under this column for two rows. How could this

Answer 1

Further to Andy Irving's answer

this worked for me (on SQL Server 2005) in a similar situation where I have a composite key and I need to swap a field which is part of the unique constraint.

key: pID, LNUM rec1: 10, 0 rec2: 10, 1 rec3: 10, 2

and I need to swap LNUM so that the result is

key: pID, LNUM rec1: 10, 1 rec2: 10, 2 rec3: 10, 0

the SQL needed:

UPDATE    DOCDATA    
SET       LNUM = CASE LNUM
              WHEN 0 THEN 1
              WHEN 1 THEN 2 
              WHEN 2 THEN 0 
          END
WHERE     (pID = 10) 
  AND     (LNUM IN (0, 1, 2))