Serial and identity columns in PostgreSQL - Step-by-Step Execution

Concept Flow - Serial and identity columns

Create Table

↓

Define Column as Serial or Identity

↓

Insert Row without specifying column

↓

Database auto-generates unique number

↓

Row stored with auto-generated value

↓

Next insert uses next number

When you create a table with a serial or identity column, PostgreSQL automatically generates unique numbers for that column on each insert if you don't specify a value.

Execution Sample

PostgreSQL

CREATE TABLE users (
  id SERIAL PRIMARY KEY,
  name TEXT
);

INSERT INTO users (name) VALUES ('Alice');
INSERT INTO users (name) VALUES ('Bob');

Creates a table with a serial id column and inserts two rows, letting the database assign id values automatically.

Execution Table

Step	Action	id value assigned	Row inserted	Notes
1	Create table with id as SERIAL	N/A	No	Table created, sequence for id starts at 1
2	Insert 'Alice' without id	1	Yes	id auto-assigned as 1
3	Insert 'Bob' without id	2	Yes	id auto-assigned as 2
4	Select all rows	N/A	Yes	Rows: (1, 'Alice'), (2, 'Bob')

💡 No more inserts; sequence values assigned automatically for each insert without id

Variable Tracker

Variable	Start	After Insert 1	After Insert 2	Final
sequence_value	0	1	2	2
table_rows	empty	(1, 'Alice')	(1, 'Alice'), (2, 'Bob')	(1, 'Alice'), (2, 'Bob')

Key Moments - 3 Insights

Why don't we specify the id value when inserting?

Because the id column is defined as SERIAL, PostgreSQL automatically generates a unique id using an internal sequence, as shown in steps 2 and 3 of the execution_table.

What happens if we insert a row and specify the id manually?

You can specify the id manually, but if you insert a value that conflicts with the sequence, it may cause errors or duplicate keys. The sequence does not automatically update to the manually inserted value.

How is SERIAL different from IDENTITY columns?

SERIAL is a shorthand that creates an integer column and a sequence behind the scenes. IDENTITY is a SQL standard way to auto-generate values with more control. Both auto-generate unique numbers, but IDENTITY is preferred in modern PostgreSQL.

Visual Quiz - 3 Questions

Look at the execution_table, what id value is assigned to the first inserted row?

A 0

B 1

C 2

D NULL

💡 Hint

Check Step 2 in the execution_table where 'Alice' is inserted.

At which step does the table contain two rows?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look at the 'table_rows' variable in variable_tracker after Insert 2.

If we manually insert a row with id = 5, what happens to the sequence value?

A Sequence updates to 5 automatically

B Sequence resets to 0

C Sequence stays at last generated value

D Sequence throws an error

💡 Hint

Refer to key_moments about manual id insertion and sequence behavior.

Concept Snapshot

Serial and Identity columns auto-generate unique numbers for a column.
SERIAL creates an integer column with an auto-incrementing sequence.
IDENTITY is the SQL standard for auto-generated columns.
Insert rows without specifying these columns to get automatic values.
Useful for primary keys to ensure uniqueness.

Full Transcript

This visual execution shows how PostgreSQL handles serial and identity columns. When you create a table with a serial column, PostgreSQL creates a sequence starting at 1. Each time you insert a row without specifying the serial column, the database assigns the next number from the sequence automatically. The execution table traces creating the table, inserting two rows, and the assigned id values. The variable tracker shows how the sequence value increments and how rows accumulate in the table. Key moments clarify why you don't specify the id manually and differences between serial and identity. The quiz tests understanding of assigned values and sequence behavior. This helps beginners see step-by-step how auto-incrementing columns work in PostgreSQL.

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