[GRASS-user] Re: Connect two tables in vector data

[Sebastian Schubert]

Thanks a lot for the explanation!

Sebastian

On 12/10/11 22:34, Moritz Lennert wrote:
> On 11/10/11 10:26, Sebastian Schubert wrote:
>>
>> On Tuesday 11 October 2011 01:17:15 Richard Chirgwin wrote:
>>> On 11/10/11 4:02 PM, grass-user-request at lists.osgeo.org wrote:
>>>> Hi,
>>>>
>>>> using 6.4.1, I imported a vector shape file which includes a
>>>> tables. This table got a "cat" column by grass (which I don't
>>>> need, I think) and already has a "key" column. In addition, I have
>>>> got another table which also has a "key" column. Using both "key"
>>>> columns, I want to combine both tables. How can I do that? I was
>>>> able connect, as a second layer, the additional table to the
>>>> vector data using the "key" column but then as far as I can see it
>>>> is not really connected to the first table.
>>>>
>>>> I guess this is a standard procedure but I was not able to find the
>>>> solution.
>>>>
>>>> Thanks
>>>> Sebastian
>>>
>>> Sebastian -
>>>
>>> I think you're asking to connect two tables to the same map?
>>
>> Yes, I think so.
>>
>>> The two tables need to refer to the same features, and you use
>>> v.db.connect
>>>
>>> Note that you can specify a different name for the key column using
>>> the key= parameter.
>>
>> I think, this is what I tried. Here is what I did:
>>
>> I imported the map baubloecke.shp. It already includes data (which is
>> then called table "baubloecke":
>>
>> GRASS 6.4.1 (Soldner):~>  v.db.connect -p baubloecke
>> Vector map<baubloecke at Berlin>  is connected by:
>> layer<1>  table<baubloecke>  in database
>> </home/schubert/grassdata/Soldner/Berlin/dbf/>  through driver<dbf>  with
>> key<cat>
>>
>> Ok, it is connected with key "cat" which has been introduced by grass, I
>> think. The table "baubloecke" includes the column I want, as well as the
>> table "bbdb":
>>
>> GRASS 6.4.1 (Soldner):~>  db.columns baubloecke
>> cat
>> ELEMNAME
>> KLASSENNAM
>> SCHLUESSEL
>> GRASS 6.4.1 (Soldner):~>  db.columns bbdb
>> SCHLUESSEL
>> BLOCKART
>> FLALLE
>> NUTZ
>> FISNUTZUNG
>> TYP
>> TYPKLAR
>> VG
>> STSTRTYP
>> STSTRNAME
>>
>> I connected bbdb to the map:
>>
>> GRASS 6.4.1 (Soldner):~>  v.db.connect -o map=baubloecke table=bbdb
>> key=SCHLUESSEL layer=2
>> The table<bbdb>  is now part of vector map<baubloecke>  and may be
>> deleted
>> or overwritten by GRASS modules
>> Select privileges were granted on the table
>> GRASS 6.4.1 (Soldner):~>  v.db.connect -p baubloecke
>> Vector map<baubloecke at Berlin>  is connected by:
>> layer<1>  table<baubloecke>  in database
>> </home/schubert/grassdata/Soldner/Berlin/dbf/>  through driver<dbf>  with
>> key<cat>
>> layer<2>  table<bbdb>  in database
>> </home/schubert/grassdata/Soldner/Berlin/dbf/>  through driver<dbf>  with
>> key<SCHLUESSEL>
>>
>> I tried to verify the connection using the wxpython gui. I opened the
>> map there (tried both layer 1 and 2). Using the "Query raster/vector
>> map(s)" tool, I was able to only get information from the layer 1 or the
>> original table when clicking on a polygon.
>>
>> Ok, maybe table 1 has to be connected also via the key "SCHLUESSEL" so I
>> tried:
>>
>> GRASS 6.4.1 (Soldner):~>  v.db.connect -o map=baubloecke
>> table=baubloecke key=SCHLUESSEL layer=1
>> ERROR: Data type of key column must be integer
>> GRASS 6.4.1 (Soldner):~>  v.db.connect -o map=baubloecke
>> table=baubloecke key=3 layer=1
>> ERROR: Missing column<3>  in table<baubloecke>
>>
>> I do not understand this... Apparently, grass does not accept the key
>> SCHLUESSEL so I tried the number of the column (?) 3. No success,
>> either.
> 
> You are mixing up two different things here:
> 
> 1) A vector map in GRASS can be (but does not have to be) connceted to
> an attribute table. Any object in the vector map can have a category
> value (sort of an identification number). If the map is connected to a
> table, this table has to contain a column which contains these category
> values. By default, this column is called 'cat' (but you can give it any
> arbitrary name). The information about which table a map is connected to
> and which column in this table contains the relevant category values is
> managed with v.db.connect.
> 
> 2) You can connect a map to different tables by using layers, but this
> is not what is needed here and is quite complex to grasp, so I'll leave
> it out here.
> 
> 3) What you are trying to do is not to connect the map to your second
> table (bbdb), but rather to link the table bbdb to the table baubloecke.
> In GRASS, the classical way to do this is to a) import bbdb into the
> database (which you seem to have done, but we have no info on how) using
> db.in.ogr and b) join the two tables using v.db.join. However, this last
> command does not suppport the dbf backend for attribute table handling.
> You, therefore, have to use another backend, such as the SQLite backend
> (see more info here:
> http://grass.osgeo.org/grass64/manuals/html64_user/databaseintro.html).
> 
> So, in summary, no need to use v.db.connect, but rather, use the SQLite
> backend and use db.in.ogr and v.db.join.
> 
> Moritz


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