ISO Country Code unique index

IF NOT EXISTS(SELECT TOP 1 1 FROM sys.indexes WHERE name = 'IX_ILC_country_iso_country_code')
  BEGIN
    CREATE UNIQUE NONCLUSTERED INDEX IX_ILC_country_iso_country_code
    ON dbo.country(iso_country_code ASC)
    WHERE iso_country_code IS NOT NULL;
  END

GO

IF NOT EXISTS (SELECT TOP 1 1 FROM ObjectMainMessages WHERE ObjectName = 'IX_ILC_country_iso_country_code' AND MessageType = 17)
    BEGIN
        DECLARE @NextMessageNo  INT

        SELECT @NextMessageNo   = ISNULL(MAX(messageno),799999)
        FROM ApplicationMessages
        WHERE MessageNo BETWEEN 800000 AND 999999

        SET @NextMessageNo = @NextMessageNo + 1

        --add main message 100093 = The &1 entered already exists.
        EXEC AddObjectMainMessageSp 'IX_ILC_country_iso_country_code', 17, 100093, 16, 'ISO Country Code UQ violation', 0

        --add the substitution text
        EXEC AddApplicationMessageSp @NextMessageNo, 'ISO Country Code', N'1033'

        --add substitution text to main message
        EXEC AddObjectBuildMessageSp 'IX_ILC_country_iso_country_code', 17, 1, @NextMessageNo
END