Working with Format in Postgres

Intro

The format function in postgresql allows you to specify a format string when selecting in a query. This function should be familiar to devs who have used the format function in C or related languages. In this article, we will learn how to use format with Postgresql.

The Syntax

The basic syntax of a Format is as follows:

SELECT FORMAT(format_string, ...arguments);

The format string has the following format

%[position][flags][width]type

Where:

• position: is the index of where to insert the argument
• flag: specifies items like justification
• width: instructions on how many characters to use
• type: is the data type such as integer, character, float, etc.

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');

An Example

To start, let's see how we can use the FORMAT function to create a string from values.

Here we use %s to specify the place of a string.

select format('Hello, %s %s', 'World', 10);

We can also use the format function to format the full name of our employees.

select 
	format('%s, %s', last_name, first_name) as full_name
from employees e;

Using Some Arguments

Let's add the width to our string. We will add 15 characters. Let's add '|' characters so we can see the boundaries.

select 
	format('|%15s|', last_name) as full_name
from employees e;

Notice how they are aligned to the right. We can align them left by using the flags option and adding a -.

select 
	format('|%-15s|', last_name) as full_name
from employees e;

Let's end by using the position option to reuse our arguments. We can use 1$ to place our first argument in multiple places. And similar for 2$ to reuse the second argument.

select 
	format('%1$s, %2$s / %2$s, %1$s', last_name, first_name) as full_name
from employees e;