Check constraint to require one of two columns

Question

I'm using SQL Server 2008 R2 and would like to make one of two columns required on a table; in other words, if the user provides a value for one, the other isn't required but the user can provide a value for both. How do you do this using check constraints? Thanks.

Update: Here's the table definition; the two columns are alternateID and securityID:

CREATE TABLE [dbo].[Employee](
    [employeeID] [int] IDENTITY(9500,1) NOT NULL,
    [alternateID] [nvarchar](50) NULL,
    [securityID] [nvarchar](50) NOT NULL,
    [firstName] [nvarchar](50) NOT NULL,
    [middleName] [nvarchar](50) NULL,
    [lastName] [nvarchar](50) NOT NULL,
    [suffix] [nvarchar](50) NULL,
    [job] [nvarchar](50) NULL,
    [organizationalUnit] [nvarchar](50) NOT NULL,
    [costCenter] [nvarchar](50) NOT NULL,
    [notes] [nvarchar](95) NULL,
    [createdDate] [datetime] NOT NULL,
 CONSTRAINT [PK_Employee] PRIMARY KEY CLUSTERED 
(
    [employeeID] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, 
ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]

Answer 1

_{Expanding on ypercube's question comments:}

The first thing you need to do is make the securityID column nullable:

ALTER TABLE dbo.Employee
ALTER COLUMN securityID nvarchar(50) NULL;

Then the required check constraint is quite straightforward:

ALTER TABLE dbo.Employee
ADD CONSTRAINT [CK dbo.Employee securityID or alternateID must not be NULL]
CHECK (securityID IS NOT NULL OR alternateID IS NOT NULL);

Regarding the 'friendly' name I chose for the check constraint, see:

Naming CHECK and UNIQUE Constraints by Greg Low.