Steve Crawford schrieb:
Mark Walker wrote:
I'm not sure what you're trying to do but, it appears that you database
design is incorrect. What you need is something like
CREATE TABLE temp_readings
(
_date Date,
temperature double,
source varchar(20),
)
No reading, no record. Are you suggesting that you would have a weekly
set of records for each row?
CREATE TABLE temp_readings
(
weekstart date,
sun double,
mon double,
tues, double
etc
)
Not such a great way to do it.
Ummm, I'm not trying to make a temperature database. I was responding to
the previous poster with an extremely simple example of usefulness of
the _concept_ of "null". I'm afraid I hadn't considered the possibility
that it would be mistaken as an example of an actual table.
But since you bring it up, simply omitting rows isn't necessarily an
option. A common scenario for weather observation is to take regular
snapshots or a bunch of measurements (air-temperature, humidity,
wind-speed, soil-temperature, leaf-wetness, UV radiation, etc.) which
can easily be represented in a table with a timestamp and a column for
each of the measurements. In a modular weather station where a specific
instrument can be out of service, one or more of those measurements
could be missing (null) for a period of time while the remaining
measurements are still being inserted.
Well I indeed have such a weather database, taking about 2 minute
snapshots of a couple of sensors. If one sensor does not respond
or is ignored due to error constraint, I just dont insert
a row: timestamp, sensor_id, sensorvalue, errorvalue
To do something usefull w/ the data you need to interpolate
anyway.
Just an example of how you can indeed avoid null values :-)
Regards
Tino
