Christian Schröder wrote:
Berend Tober wrote:
Christian Schröder wrote:
Peter Eisentraut wrote:
A first step in that direction would be to rethink the apparently
troublesome use of null values.
....Some of the limits are
only valid after a given date, whereas other limits are valid all the
time. How would you put this information into one or more tables? Of
course, I could use a special date to indicate that a limit is valid all
the time (e.g. 1970-01-01), but I don't think that this is better design
than representing this with a NULL value.
I disagree. Using "-infinity" fits your defined needs unambiguously,
except that you have to use "timestamp" data type rather than just "date"
I agree that this would be a correct model for the given application.
But wouldn't it be possible to think of a scenario where the same
problem arises? The core of my problem is that some of the records are
"more exactly" identified than some others. Some of them are identified
using one field, whereas some others need a second field to be uniquely
identified. Couldn't we construct examples for this?
Of course, if a NULL always means "unknown", then this approach doesn't
make sense. Where can I find an authorative definition of what NULL
means? As I have quoted before, according to the Wikipedia (far from
being authorative!) a NULL can also mean "not applicable".
A good read appears at
"http://www.postgresql.org/docs/techdocs.66.html";, where it says, to
echo Peter Eisentraut (one of your first responders) "Using nulls to
mean not applicable can indicate you haven't normalized correctly."
I put a lot of stock in Joe Celko's "SQL for Smarties", if you want a
more authoritative reference than Wikipedia.
