1. sql
  2. /joins
  3. /full-join



In SQL, a join operation combines rows from two or more tables based on a related column between them. There are several types of join operations, including the INNER JOIN, LEFT JOIN (also called a LEFT OUTER JOIN), RIGHT JOIN (also called a RIGHT OUTER JOIN), and FULL JOIN (also called a FULL OUTER JOIN).

A FULL JOIN (or FULL OUTER JOIN) returns all the rows from both tables. If there is no match, the result will contain NULL values for all columns of the non-matching table.


Here is an example of a FULL JOIN:

SELECT orders.id, customers.name 
FROM orders 
FULL JOIN customers 
ON orders.customer_id = customers.id;

This query will return all rows from the orders table, and all rows from the customers table based on the customer_id column. If a customer does not have a corresponding order, the orders.id column for that row will contain a NULL value.

Best Practices

Here are some best practices to keep in mind when using outer joins in SQL:

  • Use the proper syntax: The syntax for an outer join can vary depending on the type of database management system (DBMS) you are using. Make sure you are using the correct syntax for the system you are working with.

  • Use aliases for the table names: Aliases can make your query more readable and help to avoid confusion when working with multiple tables.

  • Be explicit about the join condition: Always specify the join condition explicitly in the ON clause of the query. This makes it clear what columns are being used to join the tables.

  • Consider using subqueries: In some cases, it may be more efficient to use a subquery rather than an outer join. Subqueries can help to simplify complex queries and improve performance.

  • Filter the data: Filter your data as early as possible in the query. This will help to minimize the amount of data that needs to be processed and improve performance.

  • Be careful with NULL values: Outer joins can return NULL values for non-matching rows. Be sure to handle these NULL values appropriately in your query.