Understanding What is Cross Apply in SQL Server

Hi Dev, before we dive deep into what Cross Apply is in SQL Server, let’s start with the basics of SQL Server.

Introduction to SQL Server

SQL Server is a Relational Database Management System (RDBMS) developed by Microsoft. It is widely used for storing, managing and accessing data. SQL Server provides various features and functionalities that make it stand out from other RDBMS systems like Oracle, MySQL, and PostgreSQL.

In SQL Server, there are various ways to write queries to fetch data from the database. One of the most important and frequently used methods is Join. Join is used to combine data from multiple tables based on a related column.

However, sometimes we encounter situations where we need to apply a table-valued function to each row of a table, and Join is not capable of doing so. This is where Cross Apply comes into the picture.

What is Cross Apply in SQL Server?

Cross Apply is a table operator in SQL Server that allows us to apply a table-valued function to each row of a table. It takes a table expression and a table-valued function as input and returns a table.

Cross Apply is similar to Inner Join, but it has some differences. In Inner Join, we join two tables based on a common column, but in Cross Apply, we apply a table-valued function to each row of one table and return the result for each row.

The result of Cross Apply is a table that combines the original table with the table returned by the table-valued function. The function is executed once for each row of the table, and the results are concatenated.

How Does Cross Apply Work?

Now that we have a basic understanding of what Cross Apply is, let’s take a closer look at how it works.

First, we need to define the table expression and the table-valued function that we want to apply. The table expression can be a table, view, or subquery, and the table-valued function can be any function that returns a table.

Next, we use the Cross Apply operator to apply the function to each row of the table expression. The function is executed for each row of the table expression, and the results are concatenated to form the final result set.

Example of Cross Apply in SQL Server

Let’s take an example to understand Cross Apply better. Suppose we have two tables: Sales and Products.

Sales	Products
101	1
102	2
103	3

The Sales table has two columns, SalesId and ProductId, while the Products table has two columns, ProductId and ProductName.

Now, suppose we want to retrieve the ProductName for each Sale. We can achieve this by applying the Cross Apply operator to the Products table and applying a table-valued function that returns a list of Products for each Sale.

The SQL query for this would be:

SELECT s.SalesId, p.ProductName FROM Sales s CROSS APPLY (SELECT ProductName FROM Products WHERE ProductId = s.ProductId) p

The result of this query would be:

SalesId	ProductName
101	Product A
102	Product B
103	Product C

In this example, we applied Cross Apply to the Products table and used a table-valued function that returns the ProductName for each Product in the Sales table.

FAQs on Cross Apply in SQL Server

What is the difference between Cross Apply and Inner Join?

Cross Apply is used to apply a table-valued function to each row of one table, while Inner Join is used to join two tables based on a common column.

Can we use Cross Apply with any table-valued function?

Yes, we can use Cross Apply with any table-valued function that returns a table.

In which situations should we use Cross Apply in SQL Server?

We should use Cross Apply in situations where we need to apply a table-valued function to each row of a table.

What are the benefits of using Cross Apply in SQL Server?

Cross Apply allows us to apply a table-valued function to each row of a table, which makes it easier to manipulate data and perform complex calculations.

Can we use Cross Apply with subqueries?

Yes, we can use Cross Apply with subqueries.

Conclusion

So, in conclusion, Cross Apply is a powerful tool in SQL Server that allows us to apply a table-valued function to each row of a table. It is a useful operator that can simplify complex calculations and manipulations of data. We hope this article has helped you understand what Cross Apply is and how it works.