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Overview

The OUTER JOIN or FULL OUTER JOIN returns all the records from the selected fields between the two tables (left table & right table) whether the join condition is met or not.

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Inner Join 🆚 Outer Join

The most significant difference between an INNER JOIN and an OUTER JOIN is that the INNER JOIN only returns the information from both tables which are common and related to each other. The OUTER JOIN will return all rows (matched/unmatched) from both tables.

We support table aliasing used in the OUTER JOIN clause.

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Syntax

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a) Basic Syntax

SELECT column_1, column_2...
FROM table_1
FULL OUTER JOIN table_2
ON table_1.matching_field = table2.matching_field;

In the above syntax:

  1. SELECT column_1, column_2... defines the columns from both tables where we want to display data.

  2. FROM table_1 represents the left table with table_1 in the FROM clause.

  3. FULL OUTER JOIN table_2 represents the right table with table_2 in the FULL OUTER JOIN condition.

  4. ON table_1.matching_field = table2.matching_field sets the join condition after the ON keyword with the matching field between the two tables.

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b) Syntax with an Alias

You can use an alias to refer to the table’s name. The results will stay the same. It only helps to write the query easier.

SELECT A.column_1, B.column_2...
FROM table_1 A //table_1 as A
FULL OUTER JOIN table_2 B //table_2 as B
ON A.matching_field = B.matching_field;
If there are no matched records from the joined tables, the NULL values will return in every column of the table that doesn’t have the matching record.

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Example

departments table

CREATE TABLE departments (
	department_id int,
	department_name string
);
INSERT INTO departments (department_id,department_name)
VALUES
	(1001, 'Sales'),
	(1002, 'Marketing'),
	(1003, 'HR'),
	(1004, 'Project'),
	(1005, 'Product');

SELECT * FROM departments;

It will create a departments table as shown below:

+----------------+------------------+
| department_id  | department_name  |
+----------------+------------------+
| 1001           | Sales            |
| 1002           | Marketing        |
| 1003           | HR               |
| 1004           | Project          |
| 1005           | Product          |
+----------------+------------------+

employee table

CREATE TABLE employee (
	employee_id int,
	employee_name string,
	dept_id int
);
INSERT INTO employee (
	employee_id,
	employee_name,
    dept_id
)
VALUES
	(2001,'Tony Stark', 1002),
	(2002,'Christian Bale', 1002),
	(2003,'Anne Hailey', 1003),
	(2004,'Wilson Cliff', 1004),
	(2005,'Susan Oh', 1001),
	(2006,'Julian Robert', 1001),
    (2007,'Gilbert Tom', null);

SELECT * FROM employee;

It will create an employee table as shown below:

+--------------+-------------------+------------+
| employee_id  | employee_name     | dept_id    |
+--------------+-------------------+------------+
| 2001         | Tony Stark        | 1002       |
| 2002         | Christian Bale    | 1002       |
| 2003         | Anne Hailey       | 1003       |
| 2004         | Wilson Cliff      | 1004       |
| 2005         | Susan Oh          | 1001       |
| 2006         | Julian Robert     | 1001       |
| 2007         | Gilbert Tom       | null       |
+--------------+-------------------+------------+

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Case 1: FULL OUTER JOIN

1) Based on the above tables, we can write an OUTER JOIN query as follows:

SELECT employee_name, department_name
FROM departments
FULL OUTER JOIN employee
ON departments.department_id = employee.dept_id;

2) The result will show every department with an employee and the employee who works under a specific department.

3) It also includes every department that does not have any employees and the employees who do not belong to a specific department.

+-------------------+-------------------+
| employee_name     | department_name   |
+-------------------+-------------------+
| Julian Robert     | Sales             |
| Susan Oh          | Sales             |
| Christian Bale    | Marketing         |
| Tony Stark        | Marketing         |
| Anne Hailey       | HR                |
| Wilson Cliff      | Project           |
| Gilbert Tom       | null              |
| null              | Product           |
+-------------------+-------------------+

The following Venn diagram illustrates the FULL OUTER JOIN:

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Case 2: FULL OUTER JOIN with WHERE Clause

a) Employee

  1. We can look up the department that does not have any employees by adding a WHERE clause and NULL as the following query:
SELECT employee_name, department_name
FROM departments
FULL OUTER JOIN employee
ON departments.department_id = employee.dept_id
WHERE employee_name IS NULL;
  1. The result will indicate that the Product department doesn’t have any employees 👨🏻‍💼
+------------------+--------------------+
| employee_name    | department_name    |
+------------------+--------------------+
| null             | Product            |
+------------------+--------------------+

b) Department

1) Let’s find out the employee who doesn’t belong to any department by adding a WHERE clause and NULL as the following query: 

SELECT employee_name, department_name
FROM employee
FULL OUTER JOIN departments
ON employee.dept_id = departments.department_id
WHERE department_name IS NULL;

2) The result will show that Gilbert Tom doesn’t belong to any department 👨🏻‍💼

+------------------+--------------------+
| employee_name    | department_name    |
+------------------+--------------------+
| Gilbert Tom      | null               |
+------------------+--------------------+

The following Venn diagram illustrates how the FULL OUTER JOIN works for the department and employee with a null value:

LEFT JOINRIGHT JOIN