Postgres provides enumerated types or ‘enums’ if you need to make sure some column values need to have a specific value out of a set of values. For example, if you need a column to only have values ‘Email’, ‘SMS’ and ‘Phone’, you can do this by first defining an enumerated type:

CREATE TYPE e_contact_method AS ENUM (
  'Email', 
  'Sms', 
  'Phone')

Then associate the enum to the column that needs to have a fixed set of values.

CREATE TABLE contact_method_info (
   contact_name text,
   contact_method e_contact_method,
   value text
)

Using enums

INSERT INTO contact_method_info
VALUES ('Jeff', 'Email', 'jeff@mail.com')

# select * from contact_method_info;
 contact_name | contact_method |     value     
 --------------+----------------+---------------
 Jeff         | Email          | jeff@mail.com
(1 row)

You cannot insert a value for the contactmethod column thats not in econtact_method enum.

# INSERT INTO contact_method_info VALUES ('Jeff', 'Fax', '4563456');
ERROR:  invalid input value for enum e_contact_method: "Fax"
LINE 1: INSERT INTO contact_method_info VALUES ('Jeff', 'Fax', '4563...

Viewing/Modifying enum values

You can view the list of values in an enum:

# select t.typname, e.enumlabel 
  from pg_type t, pg_enum e 
  where t.oid = e.enumtypid;

    typname      | enumlabel
  ------------------+-----------
  e_contact_method | Email
  e_contact_method | Sms
  e_contact_method | Phone
(3 rows)

You can append values to existing enums:

ALTER TYPE e_contact_method
 ADD VALUE 'Facebook' AFTER 'Phone';

# select t.typname, e.enumlabel from pg_type t, pg_enum e 
  where t.oid = e.enumtypid;
    typname      | enumlabel
------------------+-----------
e_contact_method | Email
e_contact_method | Sms
e_contact_method | Phone
e_contact_method | Facebook
(4 rows)

Values can be added anywhere in between as enums have a sort order which is the order in which the value was inserted, and it is preserved.

ALTER TYPE e_contact_method
 ADD VALUE 'Twitter' BEFORE 'Sms';

# select t.typname, e.enumlabel, e.enumsortorder from pg_type t, pg_enum e 
  where t.oid = e.enumtypid order by e.enumsortorder;
    typname      | enumlabel | enumsortorder
------------------+-----------+---------------
e_contact_method | Email     |             1
e_contact_method | Twitter   |           1.5
e_contact_method | Sms       |             2
e_contact_method | Phone     |             3
e_contact_method | Facebook  |             4
(5 rows)

At the time of this writing, Postgres does not provide a way to remove values from enums.