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 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
You can view the official docs for more information on each: https://www.postgresql.org/docs/9.5/sql-createsequence.html.
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:
Now, navigate to
http://localhost:8081/ to access phpMyAdmin. Then log in with the username
root and pass
Click the SQL tab and you are ready to go.
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');
We can start be creating a simple sequence.
CREATE SEQUENCE basicsequence;
Now we can use this sequences with the following.
Run this again and the sequence will increment.
Let's end with an example of a more complex sequence.
CREATE SEQUENCE complexsequence INCREMENT 10 MINVALUE 30 MAXVALUE 60 START 30 CYCLE;
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...