FULL OUTER JOIN: This type of join returns all the rows from both tables, whether there is a match or not. If there is no match, the result will contain NULL values. The syntax for a full outer join is:
SELECT column1, column2
FROM table1
FULL OUTER JOIN table2
ON table1.column = table2.column;
Real-life example: A company has a table of employees and a table of departments. To get a list of all employees and their department names, even if the employee does not have a department yet, you could use a full outer join to combine the data from the employees and departments tables.
SELECT employees.name, departments.department_name
FROM employees
FULL OUTER JOIN departments
ON employees.department_id = departments.department_id
There are several best practices for using full outer joins in SQL Server:
Use the ON clause to specify the join condition: The ON clause should be used to specify the condition for the join, rather than using the WHERE clause. This makes the query more readable and ensures that the join is only applied to the specified columns.
Use indexes on join columns: To improve query performance, it is important to have indexes on the columns that are used in the join condition. This allows the database engine to quickly locate the matching rows in both tables.
Use aliases for table names: Using aliases for table names can make the query more readable and make it easier to reference the correct table when there are multiple tables in the query.
Be mindful of the order of the tables: The order of the tables in the join doesn’t affect the performance of the query, but it can make the query more readable.
Watch out for NULL values: When using full outer joins, it’s important to be aware that the columns from both tables will contain NULL values if there is no match. You may need to handle these NULL values in your application or use the ISNULL or COALESCE function to replace them with a default value.
Use Explicit Columns: When joining tables, it’s best to be explicit and only select the columns that you need. This can help to reduce the amount of data that needs to be retrieved and processed, which can improve query performance.
Use FULL OUTER JOIN when it’s necessary: It’s less common to use FULL OUTER JOINs than INNER JOINs or LEFT JOINs, so use it only when it is the most appropriate choice.
Be aware of the difference between FULL OUTER JOIN, LEFT JOIN, and RIGHT JOIN, and choose the one that best fits your needs.
Example:
SELECT employees.name, departments.department_name
FROM employees
FULL OUTER JOIN departments
ON employees.department_id = departments.department_id;
In this example, the join is done on the department_id column, and only the name and department_name columns are selected.
