Wednesday, February 1, 2012

Postgresql - Domains

I'm studying good database design principles and discovered a beautiful concept : domains.

You have a SQL database and you want a column to be restrained to one of a few values.

If the list will never change, and ordering is preset or doesn't matter: use an ENUM
If the list might change but changes require programming, OR the list will not change but needs to be alpha sorted: use a DOMAIN
If the list will change, and changes do not require other programming: use a lookup table.

For instance, I defined the following domains to enforce data validation to some extent.

 CONSTRAINT max_length CHECK (LENGTH( VALUE ) < 255 )

 CONSTRAINT valid_email_form CHECK ( VALUE ~ '_*@_*._*')
 CONSTRAINT valid_email_length CHECK ( LENGTH(VALUE) > 5)
CREATE DOMAIN number_string TEXT
 CONSTRAINT valid_number CHECK (VALUE ~ '[0-9]+')
CREATE DOMAIN telephone_string TEXT
 CONSTRAINT valid_telephone_form CHECK (VALUE ~ '[0-9\-]+')
 CONSTRAINT valid_telephone_length CHECK (LENGTH(VALUE) >= 8 )

CREATE DOMAIN counter int
 CONSTRAINT positive_number CHECK ( VALUE >= 0)

Defining a table that uses them:

CREATE TABLE editoriales(
  id serial primary key,
  nombre general_text not null,
  direccion general_text,
  estado_id int,
  pais_id int,
  codigo_postal number_string , 
  telefono telephone_string

The thing that got me into their use, is that they can be treated as if they were tables ,you can modify them / add new constraints and ... they are reusable!!!

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