Oracle FULL OUTER JOIN

Summary: in this tutorial, you will learn how to use the Oracle FULL OUTER JOIN to query data from multiple tables.

Introduction to Oracle FULL OUTER JOIN clause

Suppose you have two tables T1 and T2. The following illustrates the full outer join of the two tables:

SELECT
    select_list
FROM
    T1
FULL OUTER JOIN T2 ON join_condition;

For each row in the T1 table, the full outer join compares it with every row in the T2 table.

If rows from both tables meet the join_condition, the full outer join includes columns of both rows in the result set. We say that the row in T1 table matches with the row in the T2 table in this case.

If a row from T1 table does not have any matching row from the T2 table, the full outer join will include columns from the T1 table with null values for all columns from the T2 table.

Similarly, if a row in the T2 table that does not have a matching row in the T1 table, the full outer join will include columns from T2 table with the null values for the columns in the T1 table.

Note that the OUTER keyword is optional, therefore, the FULL OUTER JOIN and FULL JOIN are the same.

This Venn diagram illustrates the full outer join of two tables:

Oracle FULL OUTER JOIN examples

First, create two tables members and projects. Suppose each member can join zero or one project, and each project can have zero or more members:

CREATE TABLE projects(
    project_id INT GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
    project_name VARCHAR2(100) NOT NULL
);

CREATE TABLE members(
    member_id INT GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
    member_name VARCHAR2(100) NOT NULL,
    project_id INT,
    FOREIGN KEY (project_id) REFERENCES projects(project_id)
);

Second, insert some projects and members into the projects and members tables:

INSERT INTO projects(project_name) 
VALUES('ERP');

INSERT INTO projects(project_name) 
VALUES('Sales CRM');

INSERT INTO members(member_name, project_id)
VALUES('John Doe',1);

INSERT INTO members(member_name, project_id)
VALUES ('Jane Doe',1);

INSERT INTO members(member_name, project_id)
VALUES ('Jack Daniel',null);

Third, use the full outer join to query data from members and projects tables:

SELECT 
    member_name, 
    project_name
FROM 
    members m
FULL OUTER JOIN projects p ON p.project_id = m.project_id
ORDER BY 
    member_name;    

Here is the output:

Jack Daniel does not join any project, Jane Doe and John Doe join the ERP project and Sales CRM project has no member.

To find the project that does not have any member, you use the following query:

SELECT 
    project_name,
    member_name
FROM 
    members m
    FULL OUTER JOIN projects p 
        ON p.project_id = m.project_id
WHERE 
    member_name IS NULL
ORDER BY 
    member_name;    

Similarly, you can find members who do not participate in any project by using the following query:

SELECT 
    member_name, 
    project_name
FROM 
    members m
    FULL OUTER JOIN projects p 
        ON p.project_id = m.project_id
WHERE 
    project_name IS NULL
ORDER BY 
    member_name;    

In this tutorial, you have learned how to use the Oracle FULL OUTER JOIN clause to query data from multiple tables.