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.