Working with Sequences in Postgres

Intro

Sequences allow you to define number generators in Postgresql. They are often use to defined primary keys on table, but are heavily customizable. In this article, we will learn how to use Sequences in Postgresql.

The Syntax

The basic syntax of a Sequences is as follows:

CREATE SEQUENCE [name];

However, there are many more parameters we can use:

CREATE [TEMP] SEQUENCE
  [IF NOT EXISTS] name
  [INCREMENT BY increment]
  [MINVALUE minvalue]
  [MAXVALUEL maxvalue]
  [START WITH start]
  [CACHE]
  [CYCLE]
  [OWNED BY table_name.column_name]

Many of the parameters are self explanatory and will be seen in our examples below. A few to note are

• cache: defines how many items need to be pre generated
• cycle: allows the sequence to wrap around, i.e. 1, 2, 3, 1, 2, 3

You can view the official docs for more information on each: https://www.postgresql.org/docs/9.5/sql-createsequence.html.

Getting Setup

We will be using docker in this article, but feel free to install your database locally instead. Once you have docker installed, create a new file called docker-compose.yml and add the following.

version: '3'
 
services:
  db:
    image: 'postgres:latest'
    ports:
      - 5432:5432
    environment:
      POSTGRES_USER: username
      POSTGRES_PASSWORD: password
      POSTGRES_DB: default_database
    volumes:
      - psqldata:/var/lib/postgresql

  phpmyadmin:
    image: phpmyadmin/phpmyadmin
    links:
      - db
    environment:
      PMA_HOST: db
      PMA_PORT: 3306
      PMA_ARBITRARY: 1
    restart: always
    ports:
      - 8081:80

volumes:
  psqldata:

Next, run docker-compose up.

Now, navigate to http://localhost:8081/ to access phpMyAdmin. Then log in with the username root and pass root_pass.

Click the SQL tab and you are ready to go.

Creating a DB

In this article, we will need some data to work with. If you don't understand these commands, don't worry, we will cover them in later articles.

We will be using the sample db provided here: https://dev.mysql.com/doc/sakila/en/. However, we will only enter what we need rather than import the whole db.

Next, let's create an film table. This is a slightly simplified version of the sakila database.

CREATE TABLE employees (
    emp_no      INT             NOT NULL,
    birth_date  DATE            NOT NULL,
    first_name  VARCHAR(14)     NOT NULL,
    last_name   VARCHAR(16)     NOT NULL,
    gender      VARCHAR(1),
    hire_date   DATE            NOT NULL,
    PRIMARY KEY (emp_no)
);

Now, let's enter a few rows

INSERT INTO employees VALUES (10001,'1953-09-02','Georgi','Facello','M','1986-06-26'),
(10002,'1964-06-02','Bezalel','Simmel','F','1985-11-21'),
(10003,'1959-12-03','Parto','Bamford','M','1986-08-28'),
(10004,'1954-05-01','Chirstian','Koblick','M','1986-12-01'),
(10005,'1955-01-21','Kyoichi','Maliniak','M','1989-09-12');

A Basic Sequence

We can start be creating a simple sequence.

CREATE SEQUENCE basicsequence;

Now we can use this sequences with the following.

SELECT nextval('basicsequence');  

Run this again and the sequence will increment.

SELECT nextval('basicsequence');  

A More Complex Sequence

Let's end with an example of a more complex sequence.

CREATE SEQUENCE complexsequence  
	INCREMENT 10
	MINVALUE 30
	MAXVALUE 60
	START 30
	CYCLE;

SELECT nextval('complexsequence');  

This sequence starts at 30, increments by 10, and has a max of 60. The sequence also cycles, so as we continue to select we will get something like 30, 40, 50, 60, 30, 40, etc...