Enumeration Type

Updated on 2024-09-02 GMT+08:00

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An enumeration (enum) type is a data type consisting of a static, ordered set of values. They are equivalent to the enum types used in many programming languages. The enumeration type can be a date in a week or a set of status values.

Declaration of Enumeration Types

Enumeration types can be created using the CREATE TYPE command. For example:

    
       CREATE TYPE mood AS ENUM ('sad', 'ok', 'happy');

After an enumeration type is created, it can be used in table and function definitions.

    
       CREATE TYPE mood AS ENUM ('sad', 'ok', 'happy');
CREATE TABLE person (name text, current_mood mood);
INSERT INTO person VALUES ('Moe', 'happy');
SELECT * FROM person WHERE current_mood = 'happy';
 name | current_mood
------+--------------
 Moe  | happy
(1 row)

Sorting

The order of enumeration values is the order of the values listed when the type is created.

    
     
       
       INSERT INTO person VALUES ('Larry', 'sad');
INSERT INTO person VALUES ('Curly', 'ok');

SELECT * FROM person WHERE current_mood > 'sad';
 name  | current_mood
-------+--------------
 Moe   | happy
 Curly | ok
(2 rows)

SELECT * FROM person WHERE current_mood > 'sad' ORDER BY current_mood;
 name  | current_mood
-------+--------------
 Curly | ok
 Moe   | happy
(2 rows)

SELECT name FROM person WHERE current_mood = (SELECT MIN(current_mood) FROM person);
 name
-------
 Larry
(1 row)

      

    
   

Security of Enumeration Types

Each enumeration data type is independent and cannot be compared with other enumeration types.

    
       CREATE TYPE happiness AS ENUM ('happy', 'very happy', 'ecstatic');
CREATE TABLE holidays (num_weeks integer, happiness happiness);
INSERT INTO holidays(num_weeks,happiness) VALUES (4, 'happy');
INSERT INTO holidays(num_weeks,happiness) VALUES (6, 'very happy');
INSERT INTO holidays(num_weeks,happiness) VALUES (8, 'ecstatic');

INSERT INTO holidays(num_weeks,happiness) VALUES (2, 'sad');
ERROR:  invalid input value for enum happiness: "sad"

SELECT person.name, holidays.num_weeks FROM person, holidays
       WHERE person.current_mood = holidays.happiness;
ERROR:  operator does not exist: mood = happiness

If comparison is required, you can use a customized operator or add an explicit type to the query.

    
        SELECT person.name, holidays.num_weeks FROM person, holidays
       WHERE person.current_mood::text = holidays.happiness::text;
 name | num_weeks
------+-----------
 Moe  |         4
(1 row)

Precautions

Enumeration tags are case sensitive. 'happy' is different from 'HAPPY'. Spaces in tags also have meanings.
Although enumeration types are primarily intended for static collections of values, there are ways to add new and renamed values to existing enumeration types (see ALTER TYPE). Existing values cannot be removed from an enumeration type, and the order of these values cannot be changed unless the enumeration type is deleted and recreated.
The mapping between enumerated values and text tags is stored in the PG_ENUM system catalog.

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