When is a good situation to use a full outer join?

Mastering SQL: When to Use a FULL OUTER JOIN

Explore the scenarios where a FULL OUTER JOIN is the most effective SQL operation for combining data from two tables, ensuring no data is lost.

In SQL, JOIN operations are fundamental for combining rows from two or more tables based on a related column between them. While INNER JOIN and LEFT/RIGHT OUTER JOIN are frequently used, the FULL OUTER JOIN often remains a mystery for many. This article will demystify the FULL OUTER JOIN, explaining its purpose, syntax, and, most importantly, when it's the optimal choice for your data retrieval needs.

Understanding the FULL OUTER JOIN

A FULL OUTER JOIN returns all rows from both the left and right tables, including rows that do not have a match in the other table. If a row in the left table does not have a match in the right table, the right-side columns will contain NULL values. Conversely, if a row in the right table does not have a match in the left table, the left-side columns will contain NULL values. This ensures that no data is discarded from either table during the join operation.

erDiagram
    CUSTOMER ||--o{ ORDER : places
    CUSTOMER { 
        int CustomerID PK
        varchar Name
    }
    ORDER { 
        int OrderID PK
        int CustomerID FK
        varchar Product
    }

Entity-Relationship Diagram for Customer and Order tables.

Key Scenarios for FULL OUTER JOIN

The FULL OUTER JOIN is particularly useful in situations where you need a complete picture of two datasets, regardless of whether they have matching records. Here are some common scenarios:

1. Comparing Two Datasets for Discrepancies: When you need to identify records that exist in one table but not the other, as well as records that exist in both. This is common for data auditing or synchronization tasks.

2. Consolidating Data from Disparate Sources: If you have two tables that contain related but not perfectly overlapping information, and you want to combine all available data into a single result set.

3. Reporting on All Entities: When a report requires showing all entities from two related lists, even if some entities only appear in one list. For example, showing all employees and all projects, even if some employees aren't assigned to projects or some projects have no assigned employees yet.

4. Identifying Missing Links: To find records that are 'orphaned' in either table, meaning they lack a corresponding entry in the other table.

CREATE TABLE Employees (
    EmployeeID INT PRIMARY KEY,
    EmployeeName VARCHAR(50)
);

CREATE TABLE Projects (
    ProjectID INT PRIMARY KEY,
    ProjectName VARCHAR(50),
    AssignedEmployeeID INT
);

INSERT INTO Employees (EmployeeID, EmployeeName) VALUES
(1, 'Alice'),
(2, 'Bob'),
(3, 'Charlie');

INSERT INTO Projects (ProjectID, ProjectName, AssignedEmployeeID) VALUES
(101, 'Project Alpha', 1),
(102, 'Project Beta', 2),
(103, 'Project Gamma', NULL),
(104, 'Project Delta', 4); -- EmployeeID 4 does not exist

SELECT
    E.EmployeeID,
    E.EmployeeName,
    P.ProjectID,
    P.ProjectName
FROM
    Employees E
FULL OUTER JOIN
    Projects P ON E.EmployeeID = P.AssignedEmployeeID;

Example of a FULL OUTER JOIN between Employees and Projects tables.

In the example above, the FULL OUTER JOIN will return:

• Alice and Project Alpha (match)
• Bob and Project Beta (match)
• Charlie and NULL Project (employee without project)
• NULL Employee and Project Gamma (project without assigned employee)
• NULL Employee and Project Delta (project assigned to a non-existent employee)

Simulating FULL OUTER JOIN in SQL Server (if not directly supported)

While most modern SQL databases like SQL Server, PostgreSQL, MySQL (8.0+), and Oracle support FULL OUTER JOIN directly, older versions of MySQL or other databases might not. In such cases, you can simulate its behavior using a combination of LEFT JOIN, RIGHT JOIN, and UNION ALL.

The logic is to:

1. Perform a LEFT JOIN to get all records from the left table and matching records from the right.
2. Perform a RIGHT JOIN to get all records from the right table and matching records from the left.
3. Use UNION ALL to combine these two result sets.
4. Filter out the duplicate matching rows that appear in both LEFT JOIN and RIGHT JOIN results (where both sides have matches).

SELECT
    E.EmployeeID,
    E.EmployeeName,
    P.ProjectID,
    P.ProjectName
FROM
    Employees E
LEFT JOIN
    Projects P ON E.EmployeeID = P.AssignedEmployeeID

UNION ALL

SELECT
    E.EmployeeID,
    E.EmployeeName,
    P.ProjectID,
    P.ProjectName
FROM
    Employees E
RIGHT JOIN
    Projects P ON E.EmployeeID = P.AssignedEmployeeID
WHERE
    E.EmployeeID IS NULL; -- Only include rows from RIGHT JOIN where there was no match in LEFT JOIN

Simulating FULL OUTER JOIN using LEFT JOIN, RIGHT JOIN, and UNION ALL.