Understanding SQL Server Cross Apply: A Comprehensive Guide for Devs

Greetings, Devs! In the world of databases, SQL Server is a popular choice for developers. It’s a powerful tool that enables you to manipulate, store, and retrieve data easily. If you’re working with large datasets, you may have encountered a scenario where a simple join operation just won’t cut it. This is where Cross Apply comes in.

What is SQL Server Cross Apply?

SQL Server Cross Apply is a powerful operator that allows you to join two tables in a way that enables you to perform complex operations on them. It’s essentially a combination of a Cross Join and an Apply operator.

With Cross Apply, you can apply a table expression to each row of another table expression. This makes it easier to perform operations on multiple rows simultaneously, without having to use a cursor or a loop. It’s a great way to optimize your queries and improve performance.

How Does Cross Apply Work?

Let’s take a look at an example. Say we have two tables: Employees and Departments.

EmployeeID	EmployeeName	DepartmentID
1	John	1
2	Jane	2
3	Bob	2
4	Sue	1

DepartmentID	DepartmentName
1	Finance
2	Marketing

If we want to join these two tables to get a list of employees and their departments, we can use a simple join:

SELECT e.EmployeeName, d.DepartmentNameFROM Employees eJOIN Departments d ON e.DepartmentID = d.DepartmentID

Result:

EmployeeName	DepartmentName
John	Finance
Jane	Marketing
Bob	Marketing
Sue	Finance

However, what if we want to get a list of all departments and the employees in each department, even if there are no employees in a given department?

Using Cross Join and Apply

This is where Cross Apply comes in handy. We’ll start by cross joining the Employees and Departments tables:

SELECT *FROM EmployeesCROSS JOIN Departments

Result:

EmployeeID	EmployeeName	DepartmentID	DepartmentID	DepartmentName
1	John	1	1	Finance
2	Jane	2	1	Finance
3	Bob	2	1	Finance
4	Sue	1	1	Finance
1	John	1	2	Marketing
2	Jane	2	2	Marketing
3	Bob	2	2	Marketing
4	Sue	1	2	Marketing

We now have a row for each possible combination of employees and departments. However, we only want the rows where the employee and department IDs match. This is where Apply comes in:

SELECT d.DepartmentName, e.EmployeeNameFROM Departments dCROSS APPLY (SELECT EmployeeNameFROM EmployeesWHERE DepartmentID = d.DepartmentID) e

Result:

DepartmentName	EmployeeName
Finance	John
Finance	Sue
Marketing	Jane
Marketing	Bob

As you can see, we now have a list of all departments, even if there are no employees in a given department. Plus, we have the list of employees within each department.

When Should You Use Cross Apply?

Cross Apply is a powerful operator that can help you optimize your queries and improve performance. However, it’s important to use it judiciously.

Here are some scenarios where Cross Apply can come in handy:

• You need to apply a table expression to every row of another table expression.
• You need to perform complex operations on multiple rows simultaneously.
• You want to optimize your queries and improve performance.

However, there are also scenarios where Cross Apply may not be the best choice:

• You’re dealing with small datasets that can be easily handled with a simple join.
• You’re working with a legacy system that may not support Cross Apply.
• Your query is already optimized and adding Cross Apply may not provide any significant performance gains.

FAQs

What’s the difference between Cross Join and Cross Apply?

Cross Join returns a result set that includes a row for every possible combination of rows from two tables, while Cross Apply applies a table expression to every row of another table expression.

Can I use Cross Apply with multiple tables?

Yes, you can use Cross Apply with multiple tables, as long as you apply a table expression to every row of another table expression.

Is Cross Apply faster than a cursor or a loop?

Yes, Cross Apply can be faster than a cursor or a loop, especially when dealing with large datasets.

Can I use Cross Apply with other operators like Outer Join or Union?

Yes, you can use Cross Apply with other operators, as long as you apply a table expression to every row of another table expression.