Serial and identity columns in PostgreSQL - Mini Project: Build & Apply

Creating a Table with Serial and Identity Columns in PostgreSQL

📖 Scenario: You are setting up a simple database table to store information about books in a library. Each book needs a unique ID that automatically increases whenever a new book is added.

🎯 Goal: Create a PostgreSQL table called books with an auto-incrementing primary key using both SERIAL and IDENTITY column types.

📋 What You'll Learn

Create a table named books with columns id_serial and id_identity as auto-incrementing primary keys.

Use SERIAL type for the id_serial column.

Use GENERATED ALWAYS AS IDENTITY for the id_identity column.

Add a title column of type VARCHAR(100) to store the book title.

💡 Why This Matters

🌍 Real World

Auto-incrementing IDs are used in databases to uniquely identify records without manual input, such as assigning unique book IDs in a library system.

💼 Career

Understanding how to create and manage auto-incrementing columns is essential for database design and is a common task for database administrators and backend developers.

1

Create the books table with a SERIAL column

Write a SQL statement to create a table called books with three columns: id_serial as a SERIAL primary key, title as VARCHAR(100), and id_identity as an integer (you will update this later).

PostgreSQL

CREATE TABLE books (
    id_serial SERIAL PRIMARY KEY,
    title VARCHAR(100),
    id_identity INTEGER
);
-- Your code here

Hint

Use SERIAL for id_serial to auto-increment IDs.

2

Add an IDENTITY column configuration

Modify the id_identity column to use GENERATED ALWAYS AS IDENTITY so it auto-increments like id_serial.

PostgreSQL

CREATE TABLE books (
    id_serial SERIAL PRIMARY KEY,
    title VARCHAR(100),
    id_identity INTEGER GENERATED ALWAYS AS IDENTITY
);
-- Your code here

Hint

Use GENERATED ALWAYS AS IDENTITY after the INTEGER type.

3

Set the primary key to the identity column

Change the table so that id_identity is the primary key instead of id_serial. Remove the primary key from id_serial.

PostgreSQL

CREATE TABLE books (
    id_serial SERIAL,
    title VARCHAR(100),
    id_identity INTEGER GENERATED ALWAYS AS IDENTITY PRIMARY KEY
);
-- Your code here

Hint

Remove PRIMARY KEY from id_serial and add it to id_identity.

4

Finalize the table with both auto-increment columns

Ensure the table books has id_serial as a SERIAL column, title as VARCHAR(100), and id_identity as an INTEGER GENERATED ALWAYS AS IDENTITY PRIMARY KEY column.

PostgreSQL

CREATE TABLE books (
    id_serial SERIAL,
    title VARCHAR(100),
    id_identity INTEGER GENERATED ALWAYS AS IDENTITY PRIMARY KEY
);
-- Your code here

Hint

Check that all columns are defined exactly as required.

Practice

1.

What is the main purpose of using SERIAL or IDENTITY columns in PostgreSQL?

easy

A. To automatically generate unique numbers for new rows

B. To store large text data efficiently

C. To create foreign key relationships

D. To improve query performance by indexing

Solution

1. Step 1: Understand the role of SERIAL and IDENTITY

   Both SERIAL and IDENTITY columns are designed to automatically generate unique numeric values for new rows in a table.

2. Step 2: Identify the main purpose

   This automatic numbering helps avoid manual entry and ensures unique identifiers for each row.

3. Final Answer:

   To automatically generate unique numbers for new rows -> Option A

4. Quick Check:

   Serial and identity columns = auto unique numbers [OK]

Hint: They auto-generate unique IDs for new rows [OK]

Common Mistakes:

• Thinking SERIAL stores text data
• Confusing SERIAL with indexing
• Assuming SERIAL creates foreign keys

2.

Which of the following is the correct syntax to create a table with an IDENTITY column named id in PostgreSQL?

CREATE TABLE users (
  id ???,
  name TEXT
);

easy

A. INT GENERATED AS IDENTITY

B. SERIAL PRIMARY KEY

C. INTEGER GENERATED ALWAYS AS IDENTITY PRIMARY KEY

D. INTEGER AUTO_INCREMENT PRIMARY KEY

Solution

1. Step 1: Recall IDENTITY syntax in PostgreSQL

   The correct syntax uses GENERATED ALWAYS AS IDENTITY for identity columns.

2. Step 2: Match the option with correct syntax

   INTEGER GENERATED ALWAYS AS IDENTITY PRIMARY KEY uses INTEGER GENERATED ALWAYS AS IDENTITY PRIMARY KEY, which is valid PostgreSQL syntax.

3. Final Answer:

   INTEGER GENERATED ALWAYS AS IDENTITY PRIMARY KEY -> Option C

4. Quick Check:

   IDENTITY syntax = GENERATED ALWAYS AS IDENTITY [OK]

Hint: IDENTITY uses GENERATED ALWAYS AS IDENTITY syntax [OK]

Common Mistakes:

• Using AUTO_INCREMENT (MySQL syntax)
• Confusing SERIAL with IDENTITY syntax
• Omitting GENERATED ALWAYS or BY DEFAULT

3.

Given the table creation:

CREATE TABLE products (
  product_id SERIAL PRIMARY KEY,
  name TEXT
);

INSERT INTO products (name) VALUES ('Pen'), ('Pencil'), ('Eraser');

SELECT product_id FROM products ORDER BY product_id;

What will be the output of the SELECT query?

medium

A. [3, 2, 1]

B. [0, 1, 2]

C. [NULL, NULL, NULL]

D. [1, 2, 3]

Solution

1. Step 1: Understand SERIAL behavior on insert

   The SERIAL column auto-increments starting from 1 for each inserted row.

2. Step 2: Check inserted rows and ordering

   Three rows inserted get product_id values 1, 2, and 3 respectively. Ordering by product_id returns them in ascending order.

3. Final Answer:

   [1, 2, 3] -> Option D

4. Quick Check:

   SERIAL starts at 1 and increments [OK]

Hint: SERIAL starts numbering at 1 by default [OK]

Common Mistakes:

• Assuming numbering starts at 0
• Expecting NULL values for SERIAL
• Thinking order is descending by default

4.

Consider this table creation:

CREATE TABLE orders (
  order_id INT GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
  product TEXT
);

Which of the following statements will cause an error when inserting data?

medium

A. INSERT INTO orders (order_id, product) VALUES (5, 'Pen'), (5, 'Pencil');

B. INSERT INTO orders (order_id, product) VALUES (5, 'Pen');

C. INSERT INTO orders (product) VALUES ('Book');

D. INSERT INTO orders (order_id, product) VALUES (DEFAULT, 'Notebook');

Solution

1. Step 1: Understand GENERATED BY DEFAULT AS IDENTITY behavior

   This allows manual insertion of order_id or automatic generation if omitted.

2. Step 2: Analyze each insert statement

   INSERT INTO orders (order_id, product) VALUES (5, 'Pen'), (5, 'Pencil'); tries to insert two rows with the same order_id = 5, violating the primary key uniqueness constraint, causing an error.

3. Final Answer:

   INSERT with duplicate order_id values causes error -> Option A

4. Quick Check:

   Duplicate primary keys cause insert errors [OK]

Hint: Duplicate primary keys cause insert errors [OK]

Common Mistakes:

• Assuming GENERATED BY DEFAULT forbids manual IDs
• Ignoring primary key uniqueness
• Thinking DEFAULT keyword causes errors

5.

You want to create a table employees with a IDENTITY column named id that automatically generates unique IDs but also allows you to insert specific IDs manually when needed. Which is the best way to define the ID column?

hard

A. Use SERIAL PRIMARY KEY for the ID column

B. Use INTEGER GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY

C. Use INTEGER GENERATED ALWAYS AS IDENTITY PRIMARY KEY

D. Use INTEGER without any auto-generation

Solution

1. Step 1: Understand difference between GENERATED ALWAYS and BY DEFAULT

   GENERATED ALWAYS AS IDENTITY disallows manual inserts; BY DEFAULT allows manual ID insertion.

2. Step 2: Match requirement to syntax

   Since manual insertion is needed sometimes, GENERATED BY DEFAULT AS IDENTITY is the best choice.

3. Final Answer:

   INTEGER GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY -> Option B

4. Quick Check:

   BY DEFAULT allows manual IDs, ALWAYS does not [OK]

Hint: BY DEFAULT allows manual ID insertion [OK]

Common Mistakes:

• Choosing GENERATED ALWAYS which forbids manual IDs
• Using SERIAL which is less flexible
• Not using any auto-generation and risking duplicates