When you create a table and you realize that you made a mistake,
or the requirements of the application change, you can drop the
table and create it again. But this is not a convenient option
if the table is already filled with data, or if the table is
referenced by other database objects (for instance a foreign key
constraint). Therefore PostgreSQL provides a family of commands
to make modifications to existing tables. Note that this is
conceptually distinct from altering the data contained in the
table: here we are interested in altering the definition, or
structure, of the table. Change default values Change column
data types All these actions are performed using the ALTER TABLE
command, whose reference page contains details beyond those
given here. To add a column, use a command like: ALTER TABLE
products ADD COLUMN description text; The new column is
initially filled with whatever default value is given (null if
you don't specify a DEFAULT clause). You can also define
constraints on the column at the sam