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:
|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.
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;
The following statement inserts a new row into the
animals table and returns the value generated for the
INSERT INTO animals(name) VALUES('Tiger') RETURNING id;