ALTER TABLE, set null in not null column, PostgreSQL 9.1

ALTER TABLE person ALTER COLUMN phone DROP NOT NULL;

More details in the manual: http://www.postgresql.org/docs/9.1/static/sql-altertable.html

Execute the command in this format

ALTER TABLE tablename ALTER COLUMN columnname SET NOT NULL;

for setting the column to not null.

Execute the command in this format:

ALTER [ COLUMN ] column { SET | DROP } NOT NULL

First, Set :
ALTER TABLE person ALTER COLUMN phone DROP NOT NULL;

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