Emulating Enumerated Data Types in PostgreSQL

There are three ways to restrict entries in a Postgres database table column. Consider a table to store "colors" where you want only 'red', 'green', or 'blue' as valid entries.

1. Enumerated data type

   CREATE TYPE valid_colors AS ENUM ('red', 'green', 'blue');
   
   CREATE TABLE t (
       color VALID_COLORS
   );
   

   Advantages are that the type can be defined once and then reused in as many tables as needed. A standard query can list all the values for an ENUM type, and can be used to make application form widgets.

   SELECT  n.nspname AS enum_schema,  
           t.typname AS enum_name,  
           e.enumlabel AS enum_value
   FROM    pg_type t JOIN 
           pg_enum e ON t.oid = e.enumtypid JOIN 
           pg_catalog.pg_namespace n ON n.oid = t.typnamespace
   WHERE   t.typname = 'valid_colors'
   
    enum_schema | enum_name     | enum_value 
   -------------+---------------+------------
    public      | valid_colors  | red
    public      | valid_colors  | green
    public      | valid_colors  | blue
   

   Disadvantages are, the ENUM type is stored in system catalogs, so a query as above is required to view its definition. These values are not apparent when viewing the table definition. And, since an ENUM type is actually a data type separate from the built in NUMERIC and TEXT data types, the regular numeric and string operators and functions don't work on it. So, one can't do a query like

   SELECT FROM t WHERE color LIKE 'bl%'; 
   

2. Check constraints

   CREATE TABLE t (
       colors TEXT CHECK (colors IN ('red', 'green', 'blue'))
   );
   

   Two advantage are that, one, "what you see is what you get," that is, the valid values for the column are recorded right in the table definition, and two, all native string or numeric operators work.

3. Foreign keys

   CREATE TABLE valid_colors (
       id SERIAL PRIMARY KEY NOT NULL,
       color TEXT
   );
   
   INSERT INTO valid_colors (colors) VALUES 
       ('red'),
       ('green'),
       ('blue');
   
   CREATE TABLE t (
       color_id INTEGER REFERENCES valid_colors (id)
   );
   

   Essentially the same as creating an ENUM type, except, the native numeric or string operators work, and one doesn't have to query system catalogs to discover the valid values. Oh, and like #1 above, this too is form-widget-friendly.