Working with Exists in MySQL
01.27.2022
Intro
The EXISTS operator is an operator that returns true or false. We can use this clause to check if there are an items in a subquery. In this article, we will learn how to use EXISTS in MySQL.
The Syntax
The basic syntax of EXISTS is as follows:
SELECT
[columns]
FROM
[table]
WHERE
EXISTS ([subquery);If there is at least one row in the subquery, EXISTS will return true, and false otherwise.
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: mysql:latest
container_name: db
environment:
MYSQL_ROOT_PASSWORD: root_pass
MYSQL_DATABASE: app_db
MYSQL_USER: db_user
MYSQL_PASSWORD: db_user_pass
ports:
- "6033:3306"
volumes:
- dbdata:/var/lib/mysql
phpmyadmin:
image: phpmyadmin/phpmyadmin
container_name: pma
links:
- db
environment:
PMA_HOST: db
PMA_PORT: 3306
PMA_ARBITRARY: 1
restart: always
ports:
- 8081:80
volumes:
dbdata: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. We will be using the sample db provided here: https://dev.mysql.com/doc/employee/en/. However, we will only enter what we need rather than import the whole db.
With the SQL tab open (or your own sql cli going), let's first create our DB and select it.
create DATABASE if not EXISTS sakila;
USE sakila;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 ENUM ('M','F') NOT NULL,
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');An Example
Let's select all salaries where there exists at least one employee matching our query. This could be useful if we are trying to find rows without a corresponding employee entry.
SELECT
salary, emp_no
FROM
salaries
WHERE
EXISTS (
SELECT emp_no
FROM employees
WHERE hire_date < '1999-01-01'
);| salary | emp_no |
|---|---|
| 60117 | 10001 |
| 62102 | 10001 |
| 66074 | 10001 |
| 66596 | 10001 |
| 66961 | 10001 |
| 71046 | 10001 |
| 74333 | 10001 |
| 75286 | 10001 |
| 75994 | 10001 |
| 76884 | 10001 |
| 80013 | 10001 |
| 81025 | 10001 |
| 81097 | 10001 |
We can also use the NOT clause to retrieve the inverse results.
SELECT
salary, emp_no
FROM
salaries
WHERE
NOT EXISTS (
SELECT emp_no
FROM employees
WHERE hire_date < '1999-01-01'
);