Joe Celko's SQL for Smarties, 3rd Edition

6.3. Context and Missing Values

Create a domain called Tricolor that is limited to the values ‘Red’, ‘White’, and ‘Blue’, and a column in a table drawn from that domain with UNIQUE constraint on it. If my table has a ‘Red’ and two NULL values in that column, I have some information about the two NULLs. I know they will be either (‘White’, ‘Blue’) or (‘Blue’, ‘White’) when their rows are resolved. This is what Chris Date calls a “distinguished NULL,” which means we have some information in it.

If my table has a ‘Red’, a ‘White’, and a NULL value in that column, can I change the last NULL to ‘Blue’ because it can only be ‘Blue’ under the rule? Or do I have to wait until I see an actual value for that row? There is no clear way to handle this in ...