Everything You Need to Know About SQL Server Outer Apply

Greetings, Dev! In this journal article, we will dive into the world of SQL Server Outer Apply. This powerful SQL feature can help you to efficiently retrieve data from related tables. If you are looking for a way to optimize your SQL Server queries, then outer apply is definitely worth learning!

What is SQL Server Outer Apply?

SQL Server Outer Apply is a SQL Server Transact-SQL operator that is used to retrieve data from a table-valued expression. Outer Apply is similar to the Join operator, but it has some distinct differences that make it a powerful tool in certain situations.

Unlike the Inner Join operator, Outer Apply returns all the rows from the left table, even if there is no corresponding row in the right table. This can be useful when you want to retrieve all the rows from the left table and match them to data in the right table if it exists.

Outer Apply also has some performance advantages over other methods of retrieving data, especially when working with large data sets. By using Outer Apply to retrieve data, you can avoid using sub-queries or other complex methods that can be slower and less efficient.

In the next few sections, we will explore the power of Outer Apply and how it can help you to optimize your SQL Server queries.

How Does SQL Server Outer Apply Work?

SQL Server Outer Apply works by taking a table-valued expression as its input and applying it to each row in the left table. The results of the expression are then joined to the corresponding row in the left table, regardless of whether there is a match in the right table.

The syntax for Outer Apply is as follows:

SELECT	…	FROM	left_table	OUTER APPLY	table-valued_expression

Here, the table-valued expression can be any expression that returns a table. This can include table-valued functions, derived tables, or sub-queries. The result of the table-valued expression is then used as the right table in the join.

It is important to note that Outer Apply will always return all the rows from the left table, even if there is no match in the right table. If there is no match, then the result of the table-valued expression will be NULL.

Examples of SQL Server Outer Apply

Example 1: Retrieving Data from Related Tables

Let’s say that you have two tables: customers and orders. Each customer can have zero, one, or many orders. You want to retrieve a list of all the customers and the total number of orders they have placed.

You could use Outer Apply to achieve this result:

SELECT	c.customer_id	c.customer_name	totals.total_orders	FROM	customers c	OUTER APPLY	(SELECT COUNT(*) AS total_orders FROM orders o WHERE o.customer_id = c.customer_id) AS totals

Here, we are joining the orders table to the customers table using Outer Apply. The table-valued expression is a sub-query that counts the number of orders each customer has placed. The result is a list of all the customers and the total number of orders they have placed, even if they have not placed any orders.

Example 2: Retrieving Data from Complex Queries

Outer Apply can also be useful when working with complex queries that involve multiple tables or sub-queries. Let’s say that you have a query that retrieves data from three tables: customers, orders, and products. You want to retrieve a list of all the customers who have placed an order for a particular product.

You could use Outer Apply to achieve this result:

SELECT	c.customer_id	c.customer_name	p.product_name	FROM	customers c	OUTER APPLY	(SELECT TOP 1 o.order_id FROM orders o JOIN order_items oi ON o.order_id = oi.order_id JOIN products p ON oi.product_id = p.product_id WHERE o.customer_id = c.customer_id AND p.product_name = ‘Product Name’) AS orders	LEFT JOIN	order_items oi ON orders.order_id = oi.order_id	LEFT JOIN	products p ON oi.product_id = p.product_id

Here, we are using Outer Apply to retrieve the order_id of the first order placed by each customer for the specified product. The table-valued expression is a complex sub-query that joins the orders, order_items, and products tables. We then join the result of the Outer Apply to the order_items and products tables to retrieve the product details.

FAQ about SQL Server Outer Apply

What is the difference between Inner Join and Outer Apply?

The main difference between Inner Join and Outer Apply is that Inner Join only returns rows that have a matching row in the right table, while Outer Apply returns all rows from the left table, even if there is no corresponding row in the right table.

What are the benefits of using Outer Apply?

Outer Apply can be more efficient than other methods of retrieving data, especially when working with large data sets. By using Outer Apply to retrieve data, you can avoid using sub-queries or other complex methods that can be slower and less efficient.

What types of expressions can be used with Outer Apply?

Any expression that returns a table can be used with Outer Apply. This includes table-valued functions, derived tables, or sub-queries.

Can Outer Apply be used with Left Join or Right Join?

Yes, Outer Apply can be used with Left Join and Right Join. However, the order of the joins can affect the results of the query, so it is important to carefully consider the order of the joins when using Outer Apply with other join operators.

What are some common use cases for Outer Apply?

Outer Apply can be useful in a variety of situations, including retrieving data from related tables, working with complex queries that involve multiple tables or sub-queries, and optimizing queries that retrieve large amounts of data.

Conclusion

SQL Server Outer Apply is a powerful tool that can help you to efficiently retrieve data from related tables. By using Outer Apply to retrieve data, you can avoid using sub-queries or other complex methods that can be slower and less efficient. If you are looking to optimize your SQL Server queries, then Outer Apply is definitely worth learning!