Working with Delete Join in Postgres
03.02.2022
Intro
The DELETE JOIN allows us to delete data from multiple tables using a join. Often we will want o connect multiple tables, then delete rows. In this article, we will learn how to use DELETE JOIN in PostgreSQL.
The Syntax
The basic syntax of DELETE JOIN is as follows:
DELETE [table1], [table2]
FROM [table1]
JOIN [table2] ON key1 = key2
WHERE [condition]This syntax will allow us to delete multiple rows based on the join and conditions we supply.
However, PSQL does not support the DELETE JOIN, so we need to emulate this query using the IN operator in a subquery.
DELETE FROM employees
WHERE emp_no IN (
SELECT emp_no FROM salaries WHERE salary < 70000
);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'),
(10002,62102,'1987-06-26','1988-06-25'),
(10003,66074,'1988-06-25','1989-06-25'),
(10004,66596,'1989-06-25','1990-06-25'),
(10005,66961,'1990-06-25','1991-06-25'),
(10006,71046,'1991-06-25','1992-06-24'),
(10007,74333,'1992-06-24','1993-06-24'),
(10008,75286,'1993-06-24','1994-06-24'),
(10009,75994,'1994-06-24','1995-06-24'),
(10010,76884,'1995-06-24','1996-06-23'),
(10011,80013,'1996-06-23','1997-06-23'),
(10012,81025,'1997-06-23','1998-06-23'),
(10013,81097,'1998-06-23','1999-06-23');Examples
Let’s do an example where we select multiple employees, joined with their salaries. We will filter based on salary then delete. This will remove all employee rows and salary rows that meet our search.
First, let’s select to see all the rows we will delete.
SELECT *
FROM employees AS e
JOIN salaries AS s on e.emp_no = s.emp_no
WHERE s.salary < 70000;| emp_no | birth_date | first_name | last_name | gender | hire_date | emp_no | salary | from_date | to_date |
|---|---|---|---|---|---|---|---|---|---|
| 10001 | 1953-09-02 | Georgi | Facello | M | 1986-06-26 | 10001 | 60117 | 1986-06-26 | 1987-06-26 |
| 10001 | 1953-09-02 | Georgi | Facello | M | 1986-06-26 | 10001 | 62102 | 1987-06-26 | 1988-06-25 |
| 10001 | 1953-09-02 | Georgi | Facello | M | 1986-06-26 | 10001 | 66074 | 1988-06-25 | 1989-06-25 |
| 10001 | 1953-09-02 | Georgi | Facello | M | 1986-06-26 | 10001 | 66596 | 1989-06-25 | 1990-06-25 |
| 10001 | 1953-09-02 | Georgi | Facello | M | 1986-06-26 | 10001 | 66961 | 1990-06-25 | 1991-06-25 |
Now, we can emulate a DELETE JOIN using a IN with a Subquery. Here we delete all employees that are in the subquery where salaries are less than 70,000.
DELETE FROM employees
WHERE emp_no IN (
SELECT emp_no FROM salaries WHERE salary < 70000
);Since we specify employee after DELETE this will delete the employee rows based on our join condition. On thing to notice is we have ON DELETE CASCADE on the salary table, thus when we delete the linked employee, the salary row will also be deleted.
SELECT * FROM salaries s;| emp_no | salary | from_date | to_date |
|---|---|---|---|
| 10006 | 71046 | 1991-06-25 | 1992-06-24 |
| 10007 | 74333 | 1992-06-24 | 1993-06-24 |
| 10008 | 75286 | 1993-06-24 | 1994-06-24 |
| 10009 | 75994 | 1994-06-24 | 1995-06-24 |
| 10010 | 76884 | 1995-06-24 | 1996-06-23 |
| 10011 | 80013 | 1996-06-23 | 1997-06-23 |
| 10012 | 81025 | 1997-06-23 | 1998-06-23 |
| 10013 | 81097 | 1998-06-23 | 1999-06-23 |