## DESCRIPTION

v.db.connect prints or sets database connection for a vector map. The user can add or remove link to attribute table on the certain layer.

## NOTE

Connection information (driver, database, table, key) is stored for each map, in the file

    <database>/<location>/<mapset>/vector/<map>/dbln

If parameters for database connection are already set with db.connect, they are taken as default values and do not need to be specified each time.

When printing database connection (p or g flag) the parameter layer is ignored, i.e. all connections are printed to the output.

Attention: Removing a vector map will also delete all tables linked to it! If you use v.db.connect to link further tables to your map, it is advisable to make a copy from those tables first and connect the copied tables to the vector map (see also v.overlay).

## EXAMPLE

Note: The default database backend setting is SQLite.

### Print database connection

Print all database connection parameters for vector map.

    v.db.connect -p map=roads

Print column types and names of table linked to vector map.

    v.db.connect -c map=roads

### Connect vector map to database (DBF driver)

Connect vector map to DBF table without or with variables.

Using default DB connection:

    v.db.connect map=vectormap table=table

Using hardcoded path to DBF directory (not recommended):

    v.db.connect map=vectormap table=table \
                 database=/home/user/grassdata/spearfish60/PERMANENT/dbf

Using variable as DBF directory definition, single quotes must be used:

    v.db.connect map=vectormap table=table \
                 database='$GISDBASE/$LOCATION_NAME/$MAPSET/dbf/'

Connect vector map layer 2 and key ID to database with variables (note: if needed, single quotes must be used for the database parameter):

    v.db.connect map=vectormap table=table layer=2 key=ID

### Connect vector map to database (SQLite driver)

Very similar to DBF driver example above.

    db.connect driver=sqlite database='$GISDBASE/$LOCATION_NAME/$MAPSET/sqlite/sqlite.db'
    db.tables -p
    v.db.connect map=vectormap table=table driver=sqlite \
                 database='$GISDBASE/$LOCATION_NAME/$MAPSET/sqlite/sqlite.db'
    v.db.connect -p map=vectormap

### Connect vector map to database (MySQL driver)

    # note: connection which requires password
    db.connect driver=mysql database="host=dbserver.foo.org,dbname=my_database"
    db.login user=joshua [password=xxx]
    # ... or enter password interactively.

    db.tables -p

    # connect external table to layer 2:
    v.db.connect map=my_map table=my_mysql_table key=baz layer=2
    v.db.connect -p my_map

### Connect vector map to database (PostgreSQL driver)

    # note: connection without password being asked
    v.db.connect map=vectormap table=table layer=1 key=oid driver=pg \
                 database="host=myserver.itc.it,dbname=mydb,user=name" \
                 table=mytable key=id

### Store geometry in GRASS but attributes in PostgreSQL

This example illustrates a mixed data storage with possibility to update attributes in an external PostgreSQL database:

    # Check current settings for attribute storage:
    db.connect -p

    # Import table from PostgreSQL to new map
    # (NOTE: output map name needs to be different from table name in
    #        case that GRASS is connected to PostgreSQL):
    v.in.db driver=pg database="host=localhost,dbname=meteo" \
            table=mytable x=lon y=lat key=cat out=mytable

    v.db.connect map=mytable -p

    # Cancel table connection between map and attribute table:
    v.db.connect map=mytable -d
    v.db.connect map=mytable -p

    # Drop table which was replicated due to import:
    db.tables -p
    echo "DROP TABLE mytable" | db.execute
    db.tables -p

    # reconnect map to table in PostgreSQL:
    v.db.connect map=mytable driver=pg database="host=localhost,dbname=meteo" \
            table=mytable key=cat

    # Now the geometry is stored in GRASS while the attributes are stored
    # in PostgreSQL.

An alternative is to create a "view" of only ID, x, y [,z] columns and to use v.in.db on this view, then connect the original table to the geometry. This will be faster if the original table is very large.

### Store geometry in GRASS but attributes in PostGIS

This example illustrated a mixed data storage with possibility top update attributes in external PostGIS database:

    # Check current settings for attribute storage:
    db.connect -p

    # Import table from PostGIS to new map
    # (NOTE: output map name needs to be different from table name in
    #        case that GRASS is connected to PostGIS):
    v.in.db driver=pg database="host=localhost,dbname=meteo" \
            table=mytable x="x(geom)" y="y(geom)" key=cat out=mytable

    v.db.connect map=mytable -p

    # Cancel table connection between map and attribute table:
    v.db.connect map=mytable -d
    v.db.connect map=mytable -p

    # Drop table which was replicated due to import:
    db.tables -p
    echo "DROP TABLE mytable" | db.execute
    db.tables -p

    # reconnect map to table in PostGIS:
    v.db.connect map=mytable driver=pg database="host=localhost,dbname=meteo" \
            table=mytable key=cat

    # Now the geometry is stored in GRASS while the attributes are stored
    # in PostGIS.

## SEE ALSO

db.connect, db.copy, db.tables, v.db.addtable, v.db.droptable, v.db.addcolumn, v.db.dropcolumn, v.external, v.in.db, v.overlay

## AUTHOR

Radim Blazek, ITC-Irst, Trento, Italy