Working with CTEs in Postgres



A Common Table Expression or CTE is a temporary table result that exists within a single query. The provide better performance and readability compared to derived tables and can self reference. In this article, we will learn how to use Common Table Expressions in PostgreSQL.

The Syntax

The basic syntax of CTE is as follows:

WITH [cte_name] as (
		EXISTS ([subquery])
SELECT * FROM [cte_name];

This syntax is only slightly different than a derived table, but comes with performance benefits.

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'
    image: 'postgres:latest'
      - 5432:5432
      POSTGRES_USER: username
      POSTGRES_PASSWORD: password
      POSTGRES_DB: default_database
      - psqldata:/var/lib/postgresql

    image: phpmyadmin/phpmyadmin
      - db
      PMA_HOST: db
      PMA_PORT: 3306
    restart: always
      - 8081:80


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: 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'),
INSERT INTO salaries VALUES (10001,60117,'1986-06-26','1987-06-26'),

An Example

Let's start with an example where we have a sub query of employees and want to select from them.

with early_employees as (
    select * from employees 
	    where hire_date < '1999-01-01'
select * from early_employees where gender = 'F';
emp_no birth_date first_name last_name gender hire_date
10002 1964-06-02 Bezalel Simmel F 1985-11-21
10006 1953-04-20 Anneke Preusig F 1989-06-02
10007 1957-05-23 Tzvetan Zielinski F 1989-02-10
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
10017 1958-07-06 Cristinel Bouloucos F 1993-08-03
10018 1954-06-19 Kazuhide Peha F 1987-04-03

We can also chain together multiple CTE using a comma after each CTE. Notice we use the WITH statement only once.

with early_employees as (
    select * from employees 
	    where hire_date < '1999-01-01'
employee_salaries as (
	select * from salaries s
		join early_employees ee on ee.emp_no = s.emp_no 
		where salary > 66000
select max(salary) from employee_salaries;