problem with expression: null values in table

Frank Warmerdam warmerdam at POBOX.COM
Mon Oct 8 11:31:12 EDT 2007

Umberto Nicoletti wrote:
> Probably when the column is null it is evaluated by OGR to the empty
> string and the expression (after replacing [HINDERCATEGORIE] with '')
> becomes ' = 1' which is invalid.

Bart / Umberto,

I can confirm that the mapogr.cpp code implementing the MapServer OGR
connection type does not check for null-ness of a field.  It just
calls the OGR GetFieldAsString() function and leaves it to OGR to do
as it wishes.  OGR will return an empty (but not NULL) string as the
representation of a NULL field value regardless of type.

> Just a side note: starting with mapserver 5 the shapefile provider
> will return 0 for null numeric columns and '' for null strings.

Like Bart, I'm a bit dubious about this approach though in the absence
of a more explicit NULL approach I suppose it is no worse than producing
an error.

MapServer attributes are always kept internally as strings, and we can't
practically have any of the item values be NULL without having to very
carefully touch a lot of code.  So, we *could* potentially add a special
string representation for null fields, and try to return this consistently
from all providers.  Then declare some particular rules to use for NULL fields
in the expression evaluator.


1) Return '*NULL*' for null field
2) Treat '*NULL*' specially in numeric calculations as being not equal to
any value or in any range.  So the only numeric test that would succeed for
*NULL* would be != (also expressable as <>).
3) Treat '*NULL*' to not match any string expression except *NULL*.

Hmm, even as I start writing this down, I can see that some additional
thought is required.  Nevertheless, if there is someone more interested
in this issue than me, it would be worth following up with an RFC and
implementation for 5.2.  I'd be happy to fix mapogr.cpp to return NULL
field values in some particular form.

Best regards,

I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at
light and sound - activate the windows |
and watch the world go round - Rush    | President OSGeo,

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