When working with SQL, the JOIN operation allows you to combine rows from two or more tables based on a related column between them. In some cases, you may encounter a scenario where you need to join tables but want to include all rows from multiple tables, even if there is no match between the related columns. This is where the UNION ALL operator comes in handy.

Understanding the UNION ALL operator

The UNION ALL operator is used to combine the result sets of two or more SELECT statements into a single result set. It does not remove duplicate rows and includes all rows from each SELECT statement.

The syntax for using the UNION ALL operator in a JOIN operation is as follows:

SELECT column_name(s) FROM table1
UNION ALL
SELECT column_name(s) FROM table2;

Example of using UNION ALL with JOIN

Let’s consider an example to better understand how to use the UNION ALL operator with the JOIN operation. Suppose we have two tables, customers and orders, with the following structure:

Table: customers

+----+----------+-----------+
| id | name     | email     |
+----+----------+-----------+
| 1  | John Doe | john@example.com |
| 2  | Jane Doe | jane@example.com |
| 3  | Michael  | michael@example.com |
+----+----------+-----------+

Table: orders

+----+------------+-----------+
| id | customer_id | product   |
+----+-------------+-----------+
| 1  | 1           | Product A |
| 2  | 2           | Product B |
+----+-------------+-----------+

To retrieve all customers and their orders, including customers who have not placed any orders, we can use the following SQL query:

SELECT customers.name, orders.product 
FROM customers
LEFT JOIN orders ON customers.id = orders.customer_id
UNION ALL
SELECT customers.name, NULL AS product
FROM customers
LEFT JOIN orders ON customers.id = orders.customer_id
WHERE orders.id IS NULL;

In this query, we perform a LEFT JOIN between the customers and orders tables to retrieve matching rows and include all rows from the customers table. The second part of the query using UNION ALL selects customers without any matching order. The NULL value for the product column is added to maintain the same structure as the first SELECT statement.

By using the UNION ALL operator, we will get a result set that combines all rows from both SELECT statements, including customers without orders.

Conclusion

The UNION ALL operator is a useful tool when you need to combine rows from multiple tables in a JOIN operation, including all rows from each table, even if there is no match between the related columns. It allows you to retrieve a comprehensive result set that includes all the information you need for your analysis or reporting.

Remember to use the appropriate JOIN type, such as LEFT JOIN, to ensure you get the desired results in combination with the UNION ALL operator.