ENUM types in PostgreSQL

Introduction

ENUM types let you store a fixed set of values in a column. This helps keep data clean and consistent.

When you want to store a list of fixed options like days of the week.

When you want to limit a column to specific values like 'small', 'medium', 'large'.

When you want to make sure users can only pick from allowed choices.

When you want to save space compared to using text columns for fixed sets.

When you want the database to help prevent invalid data entries.

Syntax

PostgreSQL

CREATE TYPE enum_name AS ENUM ('value1', 'value2', 'value3');

CREATE TABLE table_name (
  column_name enum_name
);

You create the ENUM type once, then use it in table columns.

ENUM values are stored internally as labels, not numbers.

Examples

This creates a mood type with three options and uses it in a person table.

PostgreSQL

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

CREATE TABLE person (
  name TEXT,
  current_mood mood
);

This defines sizes for t-shirts and uses the ENUM in the size column.

PostgreSQL

CREATE TYPE size AS ENUM ('small', 'medium', 'large');

CREATE TABLE tshirt (
  id SERIAL PRIMARY KEY,
  size size
);

Sample Program

This example creates an ENUM for traffic lights, inserts all values, and selects them.

PostgreSQL

CREATE TYPE traffic_light AS ENUM ('red', 'yellow', 'green');

CREATE TABLE signals (
  id SERIAL PRIMARY KEY,
  light traffic_light
);

INSERT INTO signals (light) VALUES ('red'), ('green'), ('yellow');

SELECT * FROM signals ORDER BY id;

Important Notes

You cannot insert values not listed in the ENUM.

To add new values to an ENUM, you must use ALTER TYPE.

ENUMs improve data integrity by restricting allowed values.

Summary

ENUM types store a fixed set of allowed values.

They help keep data consistent and clean.

Create ENUM types once, then use them in table columns.

Practice

1. What is the main purpose of using ENUM types in PostgreSQL?

easy

A. To restrict a column to a fixed set of allowed values

B. To store large text data efficiently

C. To create temporary tables

D. To index numeric columns faster

Solution

1. Step 1: Understand ENUM type purpose

   ENUM types define a list of allowed values for a column, ensuring data consistency.

2. Step 2: Compare with other options

   Other options describe unrelated features like text storage, temporary tables, or indexing.

3. Final Answer:

   To restrict a column to a fixed set of allowed values -> Option A

4. Quick Check:

   ENUM = fixed allowed values [OK]

Hint: ENUM limits values to a fixed list, ensuring consistency [OK]

Common Mistakes:

• Thinking ENUM stores large text data
• Confusing ENUM with temporary tables
• Assuming ENUM improves indexing speed

2. Which of the following is the correct syntax to create an ENUM type named mood with values 'happy', 'sad', and 'neutral'?

easy

A. CREATE ENUM TYPE mood AS ('happy', 'sad', 'neutral');

B. CREATE ENUM mood ('happy', 'sad', 'neutral');

C. CREATE TYPE mood ENUM ['happy', 'sad', 'neutral'];

D. CREATE TYPE mood AS ENUM ('happy', 'sad', 'neutral');

Solution

1. Step 1: Recall ENUM creation syntax

   The correct syntax is CREATE TYPE name AS ENUM (values); with values in parentheses and single quotes.

2. Step 2: Check each option

   CREATE TYPE mood AS ENUM ('happy', 'sad', 'neutral'); matches the correct syntax exactly. Others have wrong keywords or brackets.

3. Final Answer:

   CREATE TYPE mood AS ENUM ('happy', 'sad', 'neutral'); -> Option D

4. Quick Check:

   CREATE TYPE ... AS ENUM (values) [OK]

Hint: Use CREATE TYPE name AS ENUM (values) syntax [OK]

Common Mistakes:

• Using CREATE ENUM instead of CREATE TYPE
• Using square brackets instead of parentheses
• Omitting AS keyword

3. Given the ENUM type and table below, what will be the result of the query?

CREATE TYPE colors AS ENUM ('red', 'green', 'blue');
CREATE TABLE items (id SERIAL PRIMARY KEY, color colors);
INSERT INTO items (color) VALUES ('green'), ('blue'), ('red');

SELECT color FROM items ORDER BY color;

medium

A. red, blue, green

B. blue, green, red

C. green, blue, red

D. red, green, blue

Solution

1. Step 1: Understand ENUM ordering

   ENUM values are ordered by their declaration order: 'red' < 'green' < 'blue'.

2. Step 2: Apply ORDER BY on color column

   Ordering by color sorts rows as per ENUM order, so 'red', 'green', 'blue'.

3. Final Answer:

   red, green, blue -> Option A

4. Quick Check:

   ENUM order = declaration order [OK]

Hint: ENUM sorts by declared order, not alphabetically [OK]

Common Mistakes:

• Assuming alphabetical order instead of ENUM order
• Confusing insertion order with sort order
• Expecting default text sorting

4. What is wrong with the following SQL code?

CREATE TYPE status AS ENUM ('new', 'in_progress', 'done');
CREATE TABLE tasks (
  id SERIAL PRIMARY KEY,
  task_status status DEFAULT 'pending'
);

medium

A. ENUM types cannot be used as column types

B. Missing semicolon after CREATE TYPE statement

C. The default value 'pending' is not in the ENUM list

D. The SERIAL keyword is deprecated

Solution

1. Step 1: Check ENUM values and default

   ENUM 'status' has values 'new', 'in_progress', 'done'. Default 'pending' is not listed.

2. Step 2: Understand default value constraints

   Default must be one of ENUM values; otherwise, it causes an error.

3. Final Answer:

   The default value 'pending' is not in the ENUM list -> Option C

4. Quick Check:

   Default must be ENUM member [OK]

Hint: Default must be one of ENUM values [OK]

Common Mistakes:

• Assuming any string can be default
• Thinking ENUM can't be column type
• Ignoring missing semicolon errors

5. You want to add a new value 'archived' to an existing ENUM type status with values ('new', 'in_progress', 'done'). Which statement correctly adds 'archived' after 'done'?

hard

A. ALTER TYPE status ADD VALUE 'archived' BEFORE 'in_progress';

B. ALTER TYPE status ADD VALUE 'archived' AFTER 'done';

C. ALTER TYPE status ADD VALUE 'archived' BEFORE 'done';

D. ALTER TYPE status MODIFY VALUE 'archived' AFTER 'done';

Solution

1. Step 1: Recall how to add ENUM values

   PostgreSQL uses ALTER TYPE ... ADD VALUE 'new_value' [BEFORE|AFTER existing_value] syntax.

2. Step 2: Identify correct position

   To add 'archived' after 'done', use AFTER 'done'.

3. Step 3: Check options

   ALTER TYPE status ADD VALUE 'archived' AFTER 'done'; matches correct syntax and position. Others use BEFORE (wrong position) or invalid keywords.

4. Final Answer:

   ALTER TYPE status ADD VALUE 'archived' AFTER 'done'; -> Option B

5. Quick Check:

   ALTER TYPE ADD VALUE ... AFTER ... [OK]

Hint: Use ALTER TYPE ADD VALUE 'val' AFTER 'existing' to position [OK]

Common Mistakes:

• Using MODIFY instead of ADD VALUE
• Placing new value BEFORE wrong existing value
• Using wrong position like BEFORE 'done'