Working with Subquery in MySQL
01.21.2022
Intro
A Subquery is a nested query inside a query. We can often combine to queries to make more advanced conditions. For example, we can search for orders that belong to a list of customers that meet a certain criteria.
The Syntax
The basic syntax of using a Sub Query is as follows:
SELECT
[columns]
FROM
[table]
WHERE
[conditions] in (
SELECT
[columns]
FROM
[table]
WHERE
[conditions]
)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');SubQuery Example
In this example, we will grab the salaries of all employees that started before a certain date.
SELECT
salary, emp_no
FROM
salaries
WHERE
emp_no IN (
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 a sub query in the FROM clause. This is also called a derived table.
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 |