GRASS-DBF driver

# DBF driver in GRASS

## Defining the DBF driver

The DBF driver is the default driver, in theory no user interaction is required. However, if the settings should be set back from a different 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

Usually DBF tables are created by GRASS when generating a vector map with attributes (and using DBF as default 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 supported.
Type conversion is performed if necessary (experimental).

Conditions allow boolean expressions using the AND, OR and NOT operators, 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 supported 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 during 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)

Last changed: $Date: 2012-09-07 10:20:29 +0200 (Fri, 07 Sep 2012) $

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