PostgreSQL – Create Auto-increment Column using SERIAL

In PostgreSQL, a sequence is a special kind of database object that generates a sequence of integers. A sequence is often used as the primary key column in a table.
The SERIAL pseudo-type can be used to generate a sequence while creating a new table.

Syntax:
CREATE TABLE table_name(
    id SERIAL
);

In the above syntax by setting the SERIAL pseudo-type to the id column, PostgreSQL performs the following:

• First, create a sequence object and set the next value generated by the sequence as the default value for the column.
• Second, add a NOT NULL constraint to the id column because a sequence always generates an integer, which is a non-null value.
• Third, assign the owner of the sequence to the id column; as a result, the sequence object is deleted when the id column or table is dropped.

The above syntax is equivalent to the below statement:

CREATE SEQUENCE table_name_id_seq;

CREATE TABLE table_name (
    id integer NOT NULL DEFAULT nextval('table_name_id_seq')
);

ALTER SEQUENCE table_name_id_seq
OWNED BY table_name.id;

PostgreSQL provides three serial pseudo-types SMALLSERIAL, SERIAL, and BIGSERIAL with the following characteristics:

Name	Storage Size	Range
SMALLSERIA	2 bytes	1 to 32, 767
SERIAL	4 bytes	1 to 2, 147, 483, 647
BIGSERIAL	8 bytes	1 to 9, 223, 372, 036, 854, 775, 807

Now let’s look into an example for better understanding.
Example:
The following statement creates the animals table with the id column as the SERIAL column:

CREATE TABLE animals(
   id SERIAL PRIMARY KEY,
   name VARCHAR NOT NULL
);

Now we will insert a single value to the animal table as below:

INSERT INTO animals(name) 
VALUES('Dog');

We repeat the above statement wit5h a different value as below:

INSERT INTO animals(name) 
VALUES('Cat');

PostgreSQL inserted two rows into the animals table with the values for the id column are 1 and 2. To verify so use the below statement:

SELECT * FROM animals;

Output:

Example 2:
The following statement inserts a new row into the animals table and returns the value generated for the id column:

INSERT INTO animals(name) 
VALUES('Tiger')
RETURNING id; 

Output: