Vector Database Management
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This page is a work in progress.
This page is a work in progress.
Please contribute if you have experience with anything that is still poorly documented.
Vector data processing
- See the GRASS [vector data processing] help page.
The GRASS 6 vector engine supports the following databases:
- DBF: DBF is the default (local) DB used for GRASS vector attributes. It is easy to use but with the simplicity comes limited features. Such limits are 10 chars per column name and no support for SQL calculations in SELECT statements.
- SQLite (GRASS 6.1 or newer): SQLite is another local database format, but much more featureful than DBF. It basically combines the power of real SQL databases with the advantage of local data storage (no server needed). A nice tool to directly work in the SQlite database is [SQLite Database Browser].
- MySQL: [!MySQL Homepage] - support is currently under revision
- PostgreSQL: [!PostgreSQL Homepage]
- AsciiText (.csv, etc.):
- v.in.ascii module help page
- v.out.ascii module help page
- External DB support via ODBC (e.g. !FileMaker Pro)
Concepts and jargon
- Table column
- Table row
- Vector map layer
- Each vector file has a special data field named "cat" (derived originally from "category"), filled with integers, that serves to identify each vector object. The 'cat' field also serves as a "key field" that can link each vector object with a corresponding record in an attributes table of a database (NB: 'cat' values do NOT have to be unique for vector objects, but DO have to be unique in an attributes table, permitting both one-to-one and many-to-one relationships). The attributes table must contain a key field, filled with integers (only integers are permitted to serve as key fields in GRASS), that matches the values in the vector 'cat' field.
- A vector can be linked with more than one attribute table, using LAYERS. By default, every vector file has a LAYER 1 with a 'cat' field, filled with integers to identify each vector object. Additional *LAYERS*, along with their associated 'cat' fields, can be created using v.category. Each LAYER has its own independent 'cat' field that can be used to link with a separate attributes table.
- For example, a vector file of cities can have *LAYER 1* whose 'cat' field links the vector points with an attributes table of demographic data; it can also have a LAYER 2, with an associated 'cat' field (independent of the 'cat' field of LAYER 1), linked to a different attributes table of economic data. Values in the 'cat' fields of different LAYERS can be the same or different. The 'cat' values and linked attributes table can be queried independently for each LAYER. In the example, one can query the cities by population from the demographics attributes table linked with the 'cat' field of LAYER 1, or query the cities by household income from the economic attributes table linked with the same vector points through the 'cat' field of LAYER 2. _MichaelBarton - 12 Nov 2005_
Background info find in [Vector data processing in GRASS GIS].
- Connect a DB
- Copy a DB
- Copy selected columns from a DB
- Create a new table
- Create a new column
- Extract data via SQL query
- Low level access to DB
- The db.execute module
- Populate a DB
- The v.to.db module