Cross apply is a powerful feature in SQL Server that allows you to combine data from multiple tables or functions to create more complex and efficient queries. In this comprehensive guide, we’ll walk you through the ins and outs of using cross apply in SQL Server, with 15+ subheadings, examples, and expert tips to help you become a cross apply pro.

Understanding Cross Apply

Cross apply is a powerful SQL Server feature that allows you to:

• Combine data from multiple tables or functions
• Create complex and efficient queries
• Make your code more readable and maintainable

In simple terms, cross apply works by iterating through each row in the first table or query result and applying a function or subquery to that row. The result is a new row or set of rows that are combined with the original data.

When to Use Cross Apply

Cross apply is particularly useful when:

• You need to combine data from table-valued functions with other tables
• You want to simplify complex queries with multiple inline subqueries
• You want to improve query performance and readability

Cross Apply vs. Inner Join

While both cross apply and inner join are used to combine data from multiple tables, there are some key differences:

Cross Apply vs. Outer Apply

Cross apply and outer apply are similar, but with one crucial difference: outer apply returns all rows from the first table, even if there are no matching rows in the function or subquery. In contrast, cross apply only returns rows with matching data.

Working with Table-Valued Functions

Table-valued functions (TVFs) are a powerful feature in SQL Server that return a table as their output. Cross apply is an excellent way to work with TVFs, allowing you to combine their output with other tables or data sources.

Example: Using Cross Apply with a TVF

Suppose you have a TVF called GetProductSales that returns the total sales for a given product. You can use cross apply to combine this data with your Products table:

SELECT P.ProductID, P.ProductName, S.TotalSales
FROM Products P
CROSS APPLY GetProductSales(P.ProductID) AS S

This query returns a list of products along with their total sales, using the GetProductSales function to retrieve the sales data.

Using Inline Subqueries

Cross apply is also useful when working with inline subqueries, allowing you to simplify complex queries and improve readability.

Example: Using Cross Apply with an Inline Subquery

Consider a scenario where you want to find the most recent order for each customer. You can use cross apply with an inline subquery to achieve this:

SELECT C.CustomerID, C.CustomerName, O.OrderID, O.OrderDate
FROM Customers C
CROSS APPLY (
    SELECT TOP 1 *
    FROM Orders
    WHERE CustomerID = C.CustomerID
    ORDER BY OrderDate DESC
) AS O

In this example, the inline subquery retrieves the most recent order for each customer, and cross apply combines the results with the Customers table.

Combining Multiple Cross Applies

You can chain multiple cross applies together to create even more complex queries and retrieve data from multiple sources.

Example: Using Multiple Cross Applies

Imagine you need to find the top-selling product in each category. You can use multiple cross applies to achieve this:

SELECT C.CategoryID, C.CategoryName, P.ProductID, P.ProductName, S.TotalSales
FROM Categories C
CROSS APPLY (
    SELECT TOP 1 P.ProductID, P.ProductName
    FROM Products P
    WHERE P.CategoryID = C.CategoryID
    ORDER BY P.UnitsInStock DESC
) AS P
CROSS APPLY GetProductSales(P.ProductID) AS S

This query uses two cross applies: one to find the top-selling product in each category and another to retrieve the total sales for each product.

Cross Apply with Aggregated Data

Cross apply can also be used with aggregated data to calculate summary statistics, such as averages, sums, or counts.

Example: Using Cross Apply with Aggregated Data

Suppose you want to calculate the average order amount for each customer. You can use cross apply with aggregated data to achieve this:

SELECT C.CustomerID, C.CustomerName, A.AverageOrderAmount
FROM Customers C
CROSS APPLY (
    SELECT AVG(OrderAmount) AS AverageOrderAmount
    FROM Orders
    WHERE CustomerID = C.CustomerID
) AS A

In this example, the cross apply retrieves the average order amount for each customer and combines the results with the Customers table.

Optimizing Cross Apply Performance

Cross apply can improve query performance by:

• Reducing the amount of data processed
• Eliminating unnecessary joins or subqueries
• Simplifying complex queries

To optimize cross apply performance, consider the following tips:

• Use table-valued functions or inline subqueries where appropriate
• Limit the number of rows returned by the function or subquery
• Use indexes to speed up data retrieval
• Combine multiple cross applies to minimize the number of separate queries

Common Cross Apply Pitfalls

When working with cross apply, be aware of the following common pitfalls:

• Using cross apply when an inner join would suffice
• Failing to properly index the tables involved in the query
• Misunderstanding the difference between cross apply and outer apply

To avoid these pitfalls, always review your queries and ensure that you’re using cross apply correctly and efficiently.

Real-World Examples

Let’s explore some real-world examples of cross apply in action:

Example: Calculating Rolling Averages

You can use cross apply to calculate rolling averages for a time series dataset This query calculates a 3-day rolling average for each date in the TimeSeries table.

SELECT T1.Date, AVG(T2.Value) AS RollingAverage
FROM TimeSeries T1
CROSS APPLY (
    SELECT TOP 3 T2.Value
    FROM TimeSeries T2
    WHERE T2.Date <= T1.Date
    ORDER BY T2.Date DESC
) AS T2
GROUP BY T1.Date

Example: Retrieving Related Data

Cross apply can be used to retrieve related data from multiple tables: This query retrieves the most recent department for each employee. This query retrieves the most recent department for each employee.

SELECT E.EmployeeID, E.FirstName, E.LastName, D.DepartmentName
FROM Employees E
CROSS APPLY (
    SELECT TOP 1 D.DepartmentID, D.DepartmentName
    FROM Departments D
    INNER JOIN EmployeeDepartments ED ON D.DepartmentID = ED.DepartmentID
    WHERE ED.EmployeeID = E.EmployeeID
    ORDER BY ED.StartDate DESC
) AS D

Advanced Cross Apply Techniques

For advanced users, cross apply can be used in combination with other SQL Server features to create even more powerful queries:

• Use cross apply with recursive common table expressions (CTEs) to traverse hierarchical data
• Combine cross apply with window functions to calculate complex metrics or rankings
• Use cross apply to perform fuzzy matching between tables

These advanced techniques can help you solve complex problems and optimize your SQL Server queries.

Key Takeaways

Cross apply is a powerful feature in SQL Server that allows you to:

• Combine data from multiple tables or functions
• Simplify complex queries
• Improve query performance and readability

To master cross apply, remember to:

• Understand the differences between cross apply, inner join, and outer apply
• Use table-valued functions and inline subqueries effectively
• Optimize performance with proper indexing and query design

FAQ

What is cross apply in SQL Server?

Cross apply is a feature in SQL Server that allows you to combine data from multiple tables or functions by iterating through each row in the first table or query result and applying a function or subquery to that row.

When should I use cross apply instead of inner join?

Use cross apply when you need to combine data from table-valued functions, work with inline subqueries, or when the function or subquery returns multiple rows for each match.

What is the difference between cross apply and outer apply?

The main difference between cross apply and outer apply is that outer apply returns all rows from the first table, even if there are no matching rows in the function or subquery, whereas cross apply only returns rows with matching data.

Can I use cross apply with regular tables, or only with table-valued functions?

You can use cross apply with both regular tables and table-valued functions. Cross apply is particularly useful with table-valued functions, but it can also be used with inline subqueries or even just to combine data from two regular tables.

Does cross apply improve query performance?

Cross apply can improve query performance by reducing the amount of data processed, eliminating unnecessary joins or subqueries, and simplifying complex queries. However, it’s important to use cross apply correctly and efficiently, and to optimize performance with proper indexing and query design.

References

Microsoft Learn. (n.d.). FROM clause plus JOIN, APPLY, PIVOT (T-SQL) – SQL Server. Retrieved April 9, 2023, from https://learn.microsoft.com/en-us/sql/t-sql/queries/from-transact-sql?view=sql-server-ver16.