ENUM types in PostgreSQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this ENUM insertion query?

Given the ENUM type mood with values ('happy', 'sad', 'neutral'), what will be the result of this insertion?

PostgreSQL

CREATE TYPE mood AS ENUM ('happy', 'sad', 'neutral');
CREATE TABLE person (id SERIAL PRIMARY KEY, name TEXT, current_mood mood);
INSERT INTO person (name, current_mood) VALUES ('Alice', 'happy');
SELECT * FROM person;

A [{"id":1,"name":"Alice","current_mood":"happy"}]

B SyntaxError: ENUM type not recognized

C Error: invalid input value for enum mood

D [{"id":1,"name":"Alice","current_mood":null}]

💡 Hint

Remember ENUM values must be one of the predefined set.

The insertion uses a valid ENUM value 'happy', so the row is inserted correctly and returned.

🧠 Conceptual

intermediate

Which statement about ENUM types in PostgreSQL is true?

Choose the correct statement about ENUM types in PostgreSQL.

A ENUM types can be altered to add new values anywhere in the list without restrictions.

B ENUM types allow any string value without restrictions once created.

C ENUM types store values as integers internally and enforce allowed values.

D ENUM types are not supported in PostgreSQL.

💡 Hint

Think about how ENUM restricts values and stores them.

PostgreSQL ENUM types store values as integers but restrict values to the predefined set. Adding new values is possible but only at the end or with specific commands.

📝 Syntax

advanced

Which option correctly adds a new value 'excited' to an existing ENUM type 'mood'?

You have an ENUM type mood with values ('happy', 'sad', 'neutral'). Which SQL command correctly adds 'excited' as a new value?

A ALTER TYPE mood ADD VALUE 'excited';

B ALTER ENUM mood ADD 'excited';

C ALTER TYPE mood ADD 'excited' AFTER 'happy';

D ALTER TYPE mood ADD VALUE 'excited' AFTER 'happy';

💡 Hint

Check the exact syntax for adding a value at a specific position.

The correct syntax to add a new value to an ENUM type at a specific position is: ALTER TYPE mood ADD VALUE 'excited' AFTER 'happy';

❓ optimization

advanced

What is a key advantage of using ENUM types over VARCHAR for fixed sets of values?

Why might you choose an ENUM type instead of VARCHAR for a column with limited possible values?

A ENUM types use less storage and improve query performance by storing values as integers internally.

B ENUM types allow any string value, so they are more flexible than VARCHAR.

C ENUM types automatically index the column without extra commands.

D ENUM types do not enforce value restrictions, making inserts faster.

💡 Hint

Think about storage and validation differences.

ENUM types store values internally as integers referencing the allowed strings, which can save space and speed up comparisons compared to VARCHAR.

🔧 Debug

expert

Why does this INSERT fail with an error?

Given the ENUM type status with values ('new', 'in_progress', 'done'), why does this query fail? INSERT INTO tasks (id, status) VALUES (1, 'completed');

PostgreSQL

CREATE TYPE status AS ENUM ('new', 'in_progress', 'done');
CREATE TABLE tasks (id INT PRIMARY KEY, status status);
INSERT INTO tasks (id, status) VALUES (1, 'completed');

A Syntax error due to missing quotes around 'completed'.

B Error because 'completed' is not a valid value in the ENUM 'status'.

C Error because the table 'tasks' does not exist.

D No error; the row is inserted with 'completed' as status.

💡 Hint

Check if the inserted value matches the ENUM allowed values.

The ENUM type 'status' only allows 'new', 'in_progress', and 'done'. 'completed' is not in the list, so the insert fails with an error.

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'