Working with Derived Table in Postgres

Intro

A derived table is a subquery used in the FROM clause. Often we want to select a subset of a table, then do further selection on that derived table. In this article, we will learn how to use Derived Tables in PostgreSQL.

The Syntax

The basic syntax of using a Derived Table is as follows. Notice that our subquery request an Alias.

SELECT 
  [columns]
FROM
  (
    SELECT 
      [columns]
    FROM
      [table]
    WHERE
      [conditions]
  ) AS [alias]

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)
);

CREATE TABLE salaries (
    emp_no      INT             NOT NULL,
    salary      INT             NOT NULL,
    from_date   DATE            NOT NULL,
    to_date     DATE            NOT NULL,
    FOREIGN KEY (emp_no) REFERENCES employees (emp_no) ON DELETE CASCADE,
    PRIMARY KEY (emp_no, from_date)
);

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'),
(10006,'1953-04-20','Anneke','Preusig','F','1989-06-02'),
(10007,'1957-05-23','Tzvetan','Zielinski','F','1989-02-10'),
(10008,'1958-02-19','Saniya','Kalloufi','M','1994-09-15'),
(10009,'1952-04-19','Sumant','Peac','F','1985-02-18'),
(10010,'1963-06-01','Duangkaew','Piveteau','F','1989-08-24'),
(10011,'1953-11-07','Mary','Sluis','F','1990-01-22'),
(10012,'1960-10-04','Patricio','Bridgland','M','1992-12-18'),
(10013,'1963-06-07','Eberhardt','Terkki','M','1985-10-20'),
(10014,'1956-02-12','Berni','Genin','M','1987-03-11'),
(10015,'1959-08-19','Guoxiang','Nooteboom','M','1987-07-02'),
(10016,'1961-05-02','Kazuhito','Cappelletti','M','1995-01-27'),
(10017,'1958-07-06','Cristinel','Bouloucos','F','1993-08-03'),
(10018,'1954-06-19','Kazuhide','Peha','F','1987-04-03'),
(10019,'1953-01-23','Lillian','Haddadi','M','1999-04-30'),
(10020,'1952-12-24','Mayuko','Warwick','M','1991-01-26');

INSERT INTO salaries VALUES (10001,60117,'1986-06-26','1987-06-26'),
(10001,62102,'1987-06-26','1988-06-25'),
(10001,66074,'1988-06-25','1989-06-25'),
(10001,66596,'1989-06-25','1990-06-25'),
(10001,66961,'1990-06-25','1991-06-25'),
(10001,71046,'1991-06-25','1992-06-24'),
(10001,74333,'1992-06-24','1993-06-24'),
(10001,75286,'1993-06-24','1994-06-24'),
(10001,75994,'1994-06-24','1995-06-24'),
(10001,76884,'1995-06-24','1996-06-23'),
(10001,80013,'1996-06-23','1997-06-23'),
(10001,81025,'1997-06-23','1998-06-23'),
(10001,81097,'1998-06-23','1999-06-23');

Derived Table Example

Here we select the max salary of employees starting before a certain date. Notice we have to use AS to give our subquery an alias

SELECT
	MAX(salary)
	FROM (
	    SELECT salary, emp_no
		FROM salaries 
		WHERE
		  emp_no IN (
		    SELECT emp_no 
			    FROM employees 
			    WHERE hire_date < '1999-01-01'
		  )
) AS older_employees;

MAX(salary)
81097

Working with Derived Table in Postgres

01.26.2022

Intro

The Syntax

Getting Setup

Creating a DB

Derived Table Example

Working with Derived Table in MySQL

Working with Exists in MySQL