SQL constraint to have one unique value in two columns

I need to assure that a value is unique in two columns (this is not a "two columns" index issue).

Table A
Column A1       Column A2

Memphis         New York     -> ok
San Francisco   Miami        -> ok
Washington      Chicago      -> ok
Miami           Las Vegas    开发者_Python百科-> Forbidden ! Miami already exists 

Is it possible ?

My example is with cities but don't focalize on that. My real need is about generated hexadecimal ids.

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In SQL Server it is possible to enforce the uniqueness with the help of an indexed view. You will also need a numbers table (if you haven't already got one) in the same database as your Table A.

Here's my testing script with some comments:

CREATE TABLE MyNumbersTable (Value int);
-- You need at least 2 rows, by the number of columns
-- you are going to implement uniqueness on
INSERT INTO MyNumbersTable
SELECT 1 UNION ALL
SELECT 2;
GO
CREATE TABLE MyUniqueCities (  -- the main table
  ID int IDENTITY,
  City1 varchar(50) NOT NULL,
  City2 varchar(50) NOT NULL
);
GO
CREATE VIEW MyIndexedView
WITH SCHEMABINDING  -- this is required for creating an indexed view
AS
SELECT
  City = CASE t.Value    -- after supplying the numbers table
    WHEN 1 THEN u.City1  -- with the necessary number of rows
    WHEN 2 THEN u.City2  -- you can extend this CASE expression
  END                    -- to add more columns to the constraint
FROM dbo.MyUniqueCities u
  INNER JOIN dbo.MyNumbersTable t
    ON t.Value BETWEEN 1 AND 2  -- change here too for more columns
GO
-- the first index on an indexed view *must* be unique,
-- which suits us perfectly
CREATE UNIQUE CLUSTERED INDEX UIX_MyIndexedView ON MyIndexedView (City)
GO
-- the first two rows insert fine
INSERT INTO MyUniqueCities VALUES ('London', 'New York');
INSERT INTO MyUniqueCities VALUES ('Amsterdam', 'Prague');
-- the following insert produces an error, because of 'London'
INSERT INTO MyUniqueCities VALUES ('Melbourne', 'London');
GO
DROP VIEW MyIndexedView
DROP TABLE MyUniqueCities
DROP TABLE MyNumbersTable
GO

Useful reading:

• Creating Indexed Views

• The "Numbers" or "Tally" Table: What it is and how it replaces a loop.

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You need to add a constraint trigger that looks it up after insert/update.