Vector points, lines and areas usually have attribute data that are stored in DBMS. The attributes are linked to each vector object using a category number (attribute ID, usually the "cat" integer column). The category numbers are stored both in the vector geometry and the attribute table.

GRASS GIS supports various RDBMS (Relational database management system) and embedded databases. SQL (Structured Query Language) queries are directly passed to the underlying database system. The set of supported SQL commands depends on the RDMBS and database driver selected.

Database drivers

The default database driver used by GRASS GIS 7 is SQLite. GRASS GIS handles multiattribute vector data by default. The db.* set of commands provides basic SQL support for attribute management, while the v.db.* set of commands operates on vector maps.

Note: The list of available database drivers can vary in various binary distributions of GRASS GIS:

sqlite Data storage in SQLite database files (default DB backend) http://sqlite.org/

dbf Data storage in DBF files http://shapelib.maptools.org/dbf_api.html

pg Data storage in PostgreSQL RDBMS http://postgresql.org/

mysql Data storage in MySQL RDBMS http://mysql.org/

odbc Data storage via UnixODBC (PostgreSQL, Oracle, etc.) http://www.unixodbc.org/

ogr Data storage in OGR files http://gdal.org/ogr/

NOTES

Database table name restrictions

No dots allowed as SQL does not support '.' (dots) in table names.
Supported table name characters are only:
```
[A-Za-z][A-Za-z0-9_]*
```
A table name must start with a character, not a number.
Text-string matching requires the text part to be 'single quoted'. When run from the command line multiple queries should be contained in "double quotes". e.g.
```
d.vect map where="individual='juvenile' and area='beach'"
```
Attempts to use a reserved SQL word (depends on database backend) as column or table name will cause a "SQL syntax error".
An error message such as "dbmi: Protocol error" either indicates an invalid column name or an unsupported column type (then the GRASS SQL parser needs to be extended).
DBF column names are limited to 10 characters (DBF API definition).

EXAMPLES

Display of vector feature selected by attribute query

Display all vector points except for LAMAR valley and extensive trapping (brackets are superfluous in this example):

d.vect trapping_sites_points fcol=black icon=basic/diamond col=white size=13 \
    where="valley <> 'LAMAR' OR (valley = 'LAMAR' AND description = 'extensive trapping')"

Select all attributes from table where str1 column values are not 'No Name':

echo "SELECT * FROM archsites WHERE str1 <> 'No Name'" | db.select

Example of subquery expressions from a list (not supported for DBF driver):

v.db.select mysites where="id IN ('P04', 'P05')"

Example of pattern matching

# field contains string:
#  for DBF driver:
v.extract rivers out=rivers_noce where="DES LIKE 'NOCE'"
#  for SQLite driver:
v.extract rivers out=rivers_noce where="DES LIKE '%NOCE%'"

# match exactly number of characters (here: 2), does not work for DBF driver:
v.db.select mysites where="id LIKE 'P__'"

#define wildcard:
v.db.select mysites where="id LIKE 'P%'"

Example of null handling

v.db.addcolumn map=roads col="nulltest int"
v.db.update map=roads col=nulltest value=1 where="cat > 2"
d.vect roads where="nulltest is null"
v.db.update map=roads col=nulltest value=2 where="cat <= 2"

Update of attributes

Examples of complex expressions in updates (using v.db.* modules):

v.db.addcolumn map=roads col="exprtest double precision"
v.db.update map=roads col=exprtest value=cat/nulltest
v.db.update map=roads col=exprtest value=cat/nulltest+cat where=cat=1

Examples of complex expressions in updates (using db.* modules):

echo "UPDATE roads SET exprtest=null"
echo "UPDATE roads SET exprtest=cat/2" | db.execute
echo "UPDATE roads SET exprtest=cat/2+cat/3" | db.execute
echo "UPDATE roads SET exprtest=NULL WHERE cat>2" | db.execute
echo "UPDATE roads SET exprtest=cat/3*(cat+1) WHERE exprtest IS NULL" | db.execute"

Instead of creating and updating new columns with an expression, you can use the expression directly in a command:

d.vect roads where="(cat/3*(cat+1))>8"
d.vect roads where="cat>exprtest"

Example of changing a SQL type (type casting)

Note: not supported for DBF driver.

North Carolina data set: convert string column to double precision:

# first copy map into current mapset
g.copy vect=geodetic_pts,mygeodetic_pts
v.db.addcolumn mygeodetic_pts col="zval double precision"

# the 'z_value' col contains 'N/A' strings, not to be converted
v.db.update mygeodetic_pts col=zval \
            qcol="CAST(z_value AS double precision)" \
            where="z_value <> 'N/A'"

Example of concatenating fields