In SQL Server, CROSS APPLY is an operator that allows you to apply a table-valued function to each row of a table expression. It is often used in conjunction with user-defined functions to simplify complex queries or to perform calculations that would be difficult to accomplish with other SQL Server operators.

The CROSS APPLY operator works by taking each row of the table expression on its left and applying the table-valued function on its right to that row. The result is a new table that contains the original columns from the left table expression as well as any new columns that were generated by the function.

Syntax

The syntax for a CROSS APPLY statement is as follows:

SELECT column1, column2, ...
FROM table_expression
CROSS APPLY table_valued_function(table_expression.column)

Here, table_expression is the table that you want to apply the function to, and table_valued_function is the function that you want to apply. The function takes the value of the specified column from table_expression as its input.

One benefit of using CROSS APPLY is that it can help you to avoid using subqueries or temporary tables. This is because the function is applied directly to the rows of the table expression, rather than requiring a separate query to be run for each row.

Another benefit of CROSS APPLY is that it can help you to improve the performance of your queries. This is because it allows you to reuse the results of the function across multiple parts of your query, rather than requiring the function to be run separately for each part of the query.

In summary, CROSS APPLY is a powerful SQL Server operator that allows you to apply a table-valued function to each row of a table expression. It can simplify complex queries and improve query performance by avoiding subqueries and reusing function results.