Thanks, all, for the kind words!
SQL Kiwi (11/9/2011)
Good question. The explanation could have been improved very slightly:"SQL Server will use the same amount of storage for a column whether its value is NULL or a "real" value."
This is true for fixed-length types (both DATETIME and NCHAR used in the example are fixed-length) but variable length columns e.g. of VARCHAR type use no room in the data row - the null bitmap is used to distinguish between NULL and an empty string.
Good addition, Paul. For varying length data, my explanation is technically still correct, but indeed incomplete. For varying legth data, NULL takes the same amount as the shortest possible "real" value (which is teh empty string for varchar and nvarchar, and a zero-length binary string for varbinary).
Advance warning as a special service for those who take the time to read the discussion - the next question in this series (scheduled to go live in about a week) will involve some heavy arithmetic!