Working with Having in Postgres

Intro

The HAVING clause allows us to filter group rows from SQL. When using the GROUP BY clause, our groups are created from rows already filters, from using the WHERE clause. Thus, we can use the HAVING clause to filter our new group rows. In this article, we will learn how to use HAVING clause in PostgreSQL.

The Syntax

The basic syntax of using a Group By is as follows:

SELECT 
  [columns]
FROM
  [table]
WHERE
  [conditions]
GROUP BY 
  [columns]
HAVING
  [conditions];

Getting Setup

For our setup, we will use docker compose to create a Postgres database and to connect phpmyadmin. Start by copying the following into a docker compose file called docker-compose.yml

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:

We can run this file, we can use docker-compose up. One this is done, open up phpmyadmin by going to http://localhost:8081.

You can then login by leaving the host empty and using the following credentials.

POSTGRES_USER: username
POSTGRES_PASSWORD: password

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.Postgres.com/doc/sakila/en/. However, we will only enter what we need rather than import the whole db.

Next, let's create an actor table.

CREATE TABLE actor (
  actor_id smallint,
  first_name VARCHAR(45) NOT NULL,
  last_name VARCHAR(45) NOT NULL,
  last_update TIMESTAMP(0) NOT NULL DEFAULT CURRENT_TIMESTAMP,
  PRIMARY KEY  (actor_id)
) ;

And finally, let's enter a few rows.

INSERT INTO actor VALUES 
(1,'PENELOPE','GUINESS','2006-02-15 04:34:33'),
(2,'NICK','WAHLBERG','2006-02-15 04:34:33'),
(3,'ED','CHASE','2006-02-15 04:34:33'),
(4,'JENNIFER','DAVIS','2006-02-15 04:34:33'),
(5,'JOHNNY','LOLLOBRIGIDA','2006-02-15 04:34:33'),
(6,'BETTE','NICHOLSON','2006-02-15 04:34:33'),
(7,'GRACE','MOSTEL','2006-02-15 04:34:33'),
(8,'MATTHEW','JOHANSSON','2006-02-15 04:34:33'),
(9,'JOAN','JOHANSSON','2006-02-15 04:34:33')

Having Example

In this example, we create a new count using the COUNT aggregate. We can filter the groups returned by using HAVING. Here, we filter our groups by all groups with more than 1 count. We can see there are two actors with the last name, JOHANSSON.


SELECT
    last_name AS LastName,
    COUNT(*) AS LastNameCount
FROM
    actor
GROUP BY LastName
HAVING LastNameCount > 1;

LastName	LastNameCount
JOHANSSON	2

Working with Having in Postgres

01.20.2022

Intro

The Syntax

Getting Setup

Creating a DB

Having Example

Working with Having in MySQL

Working with Subquery in MySQL