How to Perform a Full Outer Join in MySQL

A full outer join in MySQL allows you to combine the results of two tables, including the unmatched rows from both tables. In other words, it returns all the rows from both tables, matching them where possible and leaving null values where there is no match. This can be useful when you want to compare data from two tables or when you need to find records that exist in one table but not in the other.

Syntax of a Full Outer Join

The syntax of a full outer join in MySQL involves using the UNION and LEFT JOIN statements. Here is the general structure:

SELECT * FROM table1
LEFT JOIN table2 ON table1.column = table2.column
UNION
SELECT * FROM table1
RIGHT JOIN table2 ON table1.column = table2.column
WHERE table1.column IS NULL

Let's break down this syntax:

- The LEFT JOIN statement retrieves all the rows from the left table (table1), along with the matching rows from the right table (table2).

- The RIGHT JOIN statement retrieves all the rows from the right table (table2), along with the matching rows from the left table (table1).

- The UNION statement combines the results of the two SELECT statements into a single result set.

- The WHERE clause filters out the rows where the column from table1 is NULL, which are the unmatched rows from the left table.

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Example: Performing a Full Outer Join

Let's consider the following two tables, customers and orders, which we want to join using a full outer join:

Table: customers

| customer_id | customer_name |
|-------------|---------------|
| 1           | John          |
| 2           | Jane          |
| 3           | Mike          |

Table: orders

| order_id | customer_id | order_date  |
|----------|-------------|-------------|
| 1        | 1           | 2022-01-01  |
| 2        | 2           | 2022-02-01  |
| 3        | 3           | 2022-03-01  |
| 4        | 4           | 2022-04-01  |

To perform a full outer join on these tables and retrieve all the rows from both tables, you can use the following query:

SELECT customers.customer_id, customers.customer_name, orders.order_id, orders.order_date
FROM customers
LEFT JOIN orders ON customers.customer_id = orders.customer_id
UNION
SELECT customers.customer_id, customers.customer_name, orders.order_id, orders.order_date
FROM customers
RIGHT JOIN orders ON customers.customer_id = orders.customer_id
WHERE customers.customer_id IS NULL

This query will return the following result:

| customer_id | customer_name | order_id | order_date |
|-------------|---------------|----------|------------|
| 1           | John          | 1        | 2022-01-01 |
| 2           | Jane          | 2        | 2022-02-01 |
| 3           | Mike          | 3        | 2022-03-01 |
| NULL        | NULL          | 4        | 2022-04-01 |

In this result, you can see that all the rows from the customers table and the orders table are included, even though there is no matching customer for the order with order_id 4.

Alternative Approach: Using UNION ALL and NULL Columns

Another approach to perform a full outer join in MySQL is by using the UNION ALL statement and adding NULL columns to match the structure of the two tables. Here is an example:

SELECT customers.customer_id, customers.customer_name, orders.order_id, orders.order_date
FROM customers
LEFT JOIN orders ON customers.customer_id = orders.customer_id
UNION ALL
SELECT customers.customer_id, customers.customer_name, orders.order_id, orders.order_date
FROM customers
RIGHT JOIN orders ON customers.customer_id = orders.customer_id
WHERE customers.customer_id IS NULL
  AND orders.order_id IS NULL

This query will produce the same result as the previous example.

Best Practices

When using a full outer join in MySQL, consider the following best practices:

- Use proper indexing: Ensure that the columns used for joining the tables are indexed to optimize the join performance.

- Be cautious with large result sets: Full outer joins can potentially produce large result sets, especially when joining large tables. Make sure to limit the number of rows returned or consider filtering the data before joining.

- Understand the data: Before performing a full outer join, thoroughly analyze the data in both tables to ensure that you are joining the correct columns and that the result set aligns with your expectations.