[GRASS-user] Copy and concatenate a vector attribute between layers

[Peter Tittmann]

Moritz,

v.db.connect -p c34_patch
Vector map <c34_patch> is connected by:
layer <1/c34_patch> table <c34_patch> in database </Users/pete/grassdata/<FOO>/project_area/sqlite/sqlite.db> through driver <sqlite> with key <cat>

So it seems that only layer 1 is connected.

When try to connect layer 2:

v.db.connect map=c34_patch at project_area table=c34_patch layer=2
ERROR: Use -o to overwrite existing link for layer <2>

I’m little unclear on the ‘table’ argument, maybe that should be a now table?

Thanks very much.

Peter
> On Dec 6, 2017, at 10:27 AM, Moritz Lennert <mlennert at club.worldonline.be> wrote:
> 
> 
> 
> Am 6. Dezember 2017 18:42:22 MEZ schrieb Peter Tittmann <ptittmann at gmail.com <mailto:ptittmann at gmail.com>>:
>> Thank you, Moritz.
>> 
>> I'm pretty familiar with SQL but can’t figure out how layer 2 is stored
>> in the database. When I `pragma table_info(<vector>)` I only see the
>> layer 1 attributes.
> 
> v.db.connect -p (or grass.script.vector_db() )will show you all connections between the map and database tables.
> 
> 
> Moritz
>> 
>> I’ll work with the GRASS approach you suggested.
>> 
>> Best,
>> Peter
>> 
>> 
>>> On Dec 1, 2017, at 11:53 PM, Moritz Lennert
>> <mlennert at club.worldonline.be> wrote:
>>> 
>>> On 02/12/17 02:03, Peter Tittmann wrote:
>>>> Hi,
>>>> I have a vector (`POINT`) which has two layers containing relevant
>> attributes (’strata’ and ’stand’). I would like to concatenate the two
>> values into a new attribute for Layer 1 (’<STRATA>_<STAND>’). I’ve been
>> struggling to sort this out for too long and am tapping out. Its
>> probably blatantly obvious but it has escaped me.
>>> 
>>> 
>>> Probably the easiest way if your not very familiar with SQL:
>>> 
>>> - Create a new column in layer 1 where you can put the info from
>> layer 2
>>> - Fill this column with v.to.db option=query query_layer=2, etc
>>> - Create a second new column to hold the concatenation
>>> - v.db.update to update this new column with the concatenation of the
>> two others (something like qcol = "STRATA || '_' || STAND")
>>> 
>>> Otherwise, if there is a common key in both tables, you can just
>> create a new column and use db.execute to launch a UPDATE TABLE
>> directly, grabbing the info from the two table and concatenating it
>> directly in the query.
>>> 
>>> Moritz
>>> 
>>> 
>>>> Thank you!
>>>> Here are some specifics.
>>>> east, north: 651849.842735, 6604949.78485
>>>> c34_patch at project_area:
>>>>  Type: Point
>>>>  Id: 21622
>>>>  Layer: 1
>>>>  Category: 21623
>>>>  Driver: sqlite
>>>>  Database:
>> /Users/pete/grassdata/PortGraham/project_area/sqlite/sqlite.db
>>>>  Table: c34_patch
>>>>  Key_column: cat
>>>>  Attributes:
>>>>    cat: 21623
>>>> *strata: 3  — STRATA*
>>>> c34_patch at project_area:
>>>>  Type: Point
>>>>  Id: 21622
>>>>  Layer: 2
>>>> *Category: 10  — STAND*
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