grass-dbf(1grass) GRASS GIS User's Manual grass-dbf(1grass)
DBF DATABASE DRIVER
The DBF driver is a file based attribute table driver.
Defining the DBF driver
The DBF driver is a file based driver, in theory no user interaction is
required. However, if the settings should be set back from a different
driver to the DBF driver, the following step is required:
# keep single quotes:
db.connect driver=dbf database=’$GISDBASE/$LOCATION_NAME/$MAPSET/dbf/’
db.connect -p
The dbf/ subdirectory in the mapset must exist or must be created by
the user.
Creating a DBF table
DBF tables are created by GRASS when generating a vector map with at-
tributes and having defined the DBF as attribute driver.
If a DBF table has to be created manually, db.execute can be used or a
spreadsheet application. Also db.copy is sometimes useful as well as
db.in.ogr to import external tables.
Supported SQL commands by DBF driver
ALTER TABLE table ADD [COLUMN] columndef
ALTER TABLE table DROP COLUMN colname
CREATE TABLE table ( columndefs )
DROP TABLE table
SELECT columns FROM table
SELECT columns FROM table WHERE condition
SELECT columns FROM table ORDER BY column
DELETE FROM table
DELETE FROM table WHERE condition
INSERT INTO table VALUES (value1[,value2,...])
INSERT INTO table ( column1[,column2,...] ) VALUES (value1[,value2,...])
UPDATE table SET assignment1[,assignment2,...]
UPDATE table SET assignment1[,assignment2,...] WHERE condition
Operators available in conditions
"=" : equal
"<" : smaller than
"<=" : smaller/equal than
">" : larger than
">=" : larger/equal than
"<>" : not equal
"~" : Substring matching (non-standard SQL)
"%" : Substring matching (limited functionality)
Arithmetic expressions using constants and field values are allowed in
condition clauses and in the RHS of assignments.
Usual precedence rules and bracketing (using ’(’ and ’)’) are sup-
ported.
Type conversion is performed if necessary (experimental).
Conditions allow boolean expressions using the AND, OR and NOT opera-
tors, with the usual precedence rules.
NULLs can be tested by ’colname IS NULL’ in conditions. The negation is
’colname NOT NULL’.
Sorting: Empty fields in a character column are sorted to the end.
LIMITATIONS OF THE DBF DRIVER
The DBF driver supports only a few SQL statements since the DBF tables
are intended for simple table storage. DBF column names are limited to
10 characters (as defined in the DBF specifications). For example,
• aggregate functions (sum, count, min, max,...) are not sup-
ported in SELECT clauses;
• mathematic functions (sin, cos, exp, log,...) are not supported
in expressions;
• SQL query with IN are not supported.
ERROR MESSAGES
An error message such as:
DBMI-DBF driver error:
SQL parser error: syntax error, unexpected NAME processing ’IN’..
indicates that an unsupported SQL statement (here, ’IN’) was used. The
only solution is to switch the DBMI backend to a real SQL engine
(SQLite, PostgreSQL, MySQL etc.). See SQL support in GRASS GIS.
An error message such as:
DBMI-DBF driver error:
SQL parser error: syntax error, unexpected DESC, expecting NAME processing ’DESC’
indicates that a column name corresponds to a reserved SQL word (here:
’DESC’). A different column name should be used. If this happens dur-
ing import with v.in.ogr, the cnames parameter can be used to assign
different column names on the fly.
SEE ALSO
db.connect, SQL support in GRASS GIS
DBF Specifications (Shapelib)
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GRASS 7.8.7 grass-dbf(1grass)
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