Constrain Partial Values in Timestamp

The following should do the trick:

CREATE TABLE MyTable (col1 FLOAT NULL, col2 NVARCHAR(30) NULL, col3 DATETIME NULL);
GO

ALTER TABLE MyTable
ADD CONSTRAINT CheckOnlyOneColumnIsNull
CHECK 
(
    ( CASE WHEN col1 IS NULL THEN 0 ELSE 1 END
    + CASE WHEN col2 IS NULL THEN 0 ELSE 1 END
    + CASE WHEN col3 IS NULL THEN 0 ELSE 1 END
    ) = 1
)
GO

I know of two ways to copy timestamp data:

• A SELECT..INTO will insert explicit values into a timestamp column, but it also requires that the destination table does not exist. That will not help in your situation.
• Convert it to a VARBINARY or VARCHAR value. This is the equivalent of the SSIS conversions to DT_BYTES/DT_WSTR you mentioned. In both cases, you can copy the values, you can manipulate the values, but you cannot reinsert them into a TIMESTAMP column.

In short, no, I don't know of a way to copy the exact same timestamp values between databases. If you need the exact value, you'd need to change the column type on the destination.

In most cases I've seen, though, the value is much less important than the sequence provided by the timestamp column. If you insert the values into the destination table in the order of the timestamp values in the source table, you should end up with the same sequence, even if the values are different.