On 26/11/08 18:09, Markus Neteler wrote:
On Tue, Nov 25, 2008 at 4:29 PM, <[EMAIL PROTECTED]> wrote:
Hi,
I have a vector layer FOO which is linked to two tables in layers 1 and 2.
The categories for each vector element are different in layer 1 and (e.g. a certain area may have
the cat value "51" in layer 1 and a cat value of "42" in layer 2).
Let's assume that layer one has a VARCHAR column containing the names of the region (e.g.
database_layer_1: 51,"Wolfenstein" database_layer_2: 42 )
If a new VARCHAR column is added to layer 2 by v.db.adcol,
how can the the names from layer 1 be copied into it?
[Goal: database_layer_1: 51,"Wolfenstein" database_layer_2: 42,"Wolfenstein" ]
Unfortunately, v.db.update seems only to work within one layer.
An UPDATE/SELECT SQL-statement will not be possible unless a table can be
produced which holds the categories for both database layers for each geometry
element.
How can this be solved ?
g.copy vect=roads,myroads --o
v.info -c myroads
v.db.addtable myroads table=extratab layer=2 columns="cat
integer,label varchar(100)"
v.db.select myroads layer=2
cat|label
-> Uffa: this should be set!! Bug in v.to.db :(
... ok, I have made some fixes in SVN (r34494 and r34495) but they don't
seem to be sufficient. See related mail on grass-dev.
IIUC, the result is actually correct since myroads does not contain any
categories in layer 2 (see my response on grass-dev).
But you got us on the right track: there actually is a way to copy cats
of one layer to another layer (e.g. here from layer 1 to layer 2):
v.db.addcol FOO layer=2 col='cat1 int'
v.to.db FOO layer=2 col=cat1 option=query qlayer=1 qcol=cat
Only if any of the two layers do not have a table attached do you need
the v.db.addtable above, and to make that work you actually need
categories in both layers (which isn't the case in Markus' example)...
Now you need a join operation (db.execute). Note that this isn't
supported with DBF driver:
# I use SQLIte here:
v.db.connect -p myroads
Vector map <[EMAIL PROTECTED]> is connected by:
layer <1> table <myroads> in database
</home/neteler/grassdata/spearfish60/sqlite/sqlite.db> through driver
<sqlite> with key <cat>
layer <2> table <extratab> in database
</home/neteler/grassdata/spearfish60/sqlite/sqlite.db> through driver
<sqlite> with key <cat>
# bug workaround:
db.droptable extratab -f
db.copy from_table=myroads to_table=extratab
v.db.select myroads layer=2
cat|label
0|no data
1|interstate
2|primary highway, hard surface
3|secondary highway, hard surface
4|light-duty road, improved surface
5|unimproved road
This "works" because v.db.select selects all tuples in the table
irrespective of whether these tuples have any corresponding features in
the layer.
# simulate almost empty table (note that I fixed v.db.dropcol right
now for layer=2):
v.db.dropcol myroads layer=2 column=label
v.info -c myroads layer=2
# add new column with desired name:
v.db.addcol myroads layer=2 col="names varchar(100)"
v.info -c myroads layer=2
Displaying column types/names for database connection of layer 2:
INTEGER|cat
CHARACTER|names
v.db.select myroads layer=2
cat|names
0|
1|
2|
3|
4|
5|
# now JOIN the names into:
echo "UPDATE extratab SET names=(SELECT label FROM myroads WHERE
extratab.cat=myroads.cat);" | db.execute
# voilà:
v.db.select myroads layer=2
cat|names
0|no data
1|interstate
2|primary highway, hard surface
3|secondary highway, hard surface
4|light-duty road, improved surface
5|unimproved road
Try clicking on any of the roads : do you actually see these results in
layer 2 ?
TODO: fix v.to.db for layer>1.
It works AFAIU, you just need to use the query option.
Moritz
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