NAME¶
sql - SQL support introduction
SQL support introduction
SQL support in GRASS GIS¶
GRASS can use 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 list of available drivers can vary in various binary distributions of GRASS.
| dbf | DBF files. Data are stored in DBF files (GRASS 6 default DB backend). |
http://shapelib.maptools.org/dbf_api.html
| sqlite | SQLite embedded database. |
http://sqlite.org/
| pg | PostgreSQL RDBMS. |
http://postgresql.org/
| mysql | MySQL RDBMS. |
http://mysql.org/
| mesql | MySQL embedded database. |
http://mysql.org/
| odbc | UnixODBC. (PostgreSQL, Oracle, etc.) |
http://www.unixodbc.org/
NOTES¶
-
SQL does not support '.' (dots) in table names.
-
Supported table name characters are only:
</div>
-
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'"
-
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).
-
Attempts to use a reserved SQL word (depends on database backend) as column
or table name will cause a "SQL syntax error".
EXAMPLES¶
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 (does not work 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.addcol 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"
Examples of complex expressions in updates (using v.db.* modules):
v.db.addcol 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, does not work for DBF
driver):
# North Carolina data set: convert string column to double precision
# copy map into current mapset
g.copy vect=geodetic_pts,mygeodetic_pts
v.db.addcol 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 (does not work for DBF driver):
v.db.update vectormap column=column3 qcolumn="column1 || column2"
SEE ALSO¶
db.select, db.execute, v.db.select,
v.db.update
Database management in GRASS GIS, Help pages for database
modules
Last changed: $Date: 2014-02-15 23:27:05 +0100 (Sat, 15 Feb 2014) $
Main index - database index - full index
© 2008-2014 GRASS Development Team
