v.db.addtable will only insert category values into the table for those features which actually have a category value in the relevant layer. The use can add category values automatically by using v.category or manually with wxGUI vector digitizer before running v.db.addtable. Or one can run v.db.addtable first and then use either a combinatino of v.category + v.to.db or wxGUI vector digitizer to add the relevant lines to the table.
The supported types of columns depend on the database backend. However, all backends should support VARCHAR, INT, DOUBLE PRECISION and DATE.
The existing database connection(s) can be verified with v.db.connect.
g.copy vect=roadsmajor,myroads v.db.addtable myroads columns="slope double precision" v.db.connect -p myroads v.info -c myroads
Adding a new attribute table with two columns to layer 2:
g.copy vect=roadsmajor,myroads v.db.addtable myroads columns="slope double precision, roadname varchar(15)" layer=2 v.db.connect -p myroads v.info -c myroads v.info -c myroads layer=2
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