NAME

       v.db.update  - Updates a column in the attribute table connected to a vector map.

KEYWORDS

       vector, attribute table, database, attribute update, type casting

SYNOPSIS

       v.db.update
       v.db.update --help
       v.db.update   map=name   layer=string  column=name   [value=string]    [query_column=name]
       [where=sql_query]   [sqliteextra=name]   [--help]  [--verbose]  [--quiet]  [--ui]

   Flags:
       --help
           Print usage summary

       --verbose
           Verbose module output

       --quiet
           Quiet module output

       --ui
           Force launching GUI dialog

   Parameters:
       map=name [required]
           Name of vector map
           Or data source for direct OGR access

       layer=string [required]
           Layer number or name
           Vector features can have category values in different layers. This  number  determines
           which layer to use. When used with direct OGR access this is the layer name.
           Default: 1

       column=name [required]
           Name of attribute column to update

       value=string
           Literal value to update the column with

       query_column=name
           Name of other attribute column to query, can be combination of columns (e.g. co1+col2)

       where=sql_query
           WHERE conditions of SQL statement without ’where’ keyword
           Example: income < 1000 and population >= 10000

       sqliteextra=name
           Name of SQLite extension file for extra functions (SQLite backend only)

DESCRIPTION

       v.db.update  assigns  a  new value to a column in the attribute table connected to a given
       map. The value parameter allows updating with a literal  value.  Alternatively,  with  the
       qcol parameter values can be copied from another column in the table or be the result of a
       combination or transformation of other columns.

NOTES

       v.db.update is just a front-end to db.execute to allow easier usage.

       For complex SQL UPDATE statements, db.execute should be used.

EXAMPLES

   Replacing of NULL values
       In this example, selectively display lakes without (blue) and with NULL (red) are shown to
       find  out  which  type  is  undefined.  In  the original map there are lakes missing FTYPE
       attribute which are wetlands along streams. These NULL attributes are  replaced  with  the
       landuse type WETLAND:
       g.copy vect=lakes,mylakes
       v.db.select mylakes
       v.db.select mylakes where="FTYPE IS NULL"
       # display the lakes, show undefined FTYPE lakes in red
       g.region vector=mylakes
       d.mon wx0
       d.vect mylakes where="FTYPE NOT NULL" type=area col=blue
       d.vect mylakes where="FTYPE IS NULL" type=area col=red
       # replace NULL with FTYPE WETLAND
       v.db.update mylakes col=FTYPE value=WETLAND \
                   where="FTYPE IS NULL"
       v.db.select mylakes

   Updating of columns with on the fly calculation
       Spearfish example: adding new column, copying values from another table column with on the
       fly calculation:
       g.copy vect=fields,myfields
       v.db.addcolumn myfields col="polynum integer"
       v.db.update myfields col=polynum qcol="cat*2"
       v.db.select myfields

   Type casting
       Type cast (type conversion) of strings to double precision (unsupported by DBF driver):
       g.copy vect=geodetic_pts,mygeodetic_pts
       v.db.update mygeodetic_pts col=zval qcol="CAST(z_value AS double precision)" \
                   where="z_value <> ’N/A’"

   Updating of columns with on the fly calculation (SQLite extended functions)
       Note: this requires SQLite  extended  functions.  For  details  see  the  GRASS  GIS  Wiki
       (compilation of libsqlitefunctions.so and libsqlitefunctions.dll).

       North  Carolina  data  set  example:  adding new column, copying values from another table
       column with on the fly calculation:
       g.copy vect=precip_30ynormals,myprecip_30ynormals
       v.db.addcolumn myprecip_30ynormals column="logjuly double precision"
       v.db.update myprecip_30ynormals column="logjuly" query_column="log(jul)" \
         sqliteextra=$HOME/sqlite_extensions/libsqlitefunctions.so
       v.db.select myprecip_30ynormals columns=jul,logjuly
       jul|logjuly
       132.842|4.88916045210132
       127|4.84418708645859
       124.206|4.82194147751127
       104.648|4.65060233738593
       98.298|4.58800368106618
       ...

SEE ALSO

         db.execute,  v.db.addcolumn,  v.db.addtable,  v.db.connect,  v.db.droptable,  v.db.join,
       v.db.select

        GRASS SQL interface

AUTHOR

       Moritz Lennert (mlennert@club.worldonline.be)

SOURCE CODE

       Available at: v.db.update source code (history)

       Accessed: Thursday Aug 01 11:31:53 2024

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       © 2003-2024 GRASS Development Team, GRASS GIS 8.4.0 Reference Manual