Cross apply is a type of join operation in SQL that is used to combine the results of two tables based on a related condition between them. In this article
we will explore what cross apply is
how it works
and when to use it in your SQL queries.
To understand cross apply
let us first look at the basic concept of joins in SQL. A join is used to combine rows from two or more tables based on a related column between them. There are different types of joins such as inner join
left join
right join
and full outer join.
Cross apply is a specific type of join operation that is used in conjunction with table-valued functions. It is similar to inner join but with some key differences. Cross apply is used to apply a table-valued function to each row of the outer table and return the results as a new column. This allows you to perform complex calculations or transformations on each row of the outer table based on the related rows in the inner table.
The syntax for cross apply is as follows:
```
SELECT columns
FROM table1
CROSS APPLY table-valued-function(table1.column)
```
In this syntax
table1 is the outer table
and table-valued-function is the function that will be applied to each row of table1. The result of the function is then returned as a new column in the output.
One common use case for cross apply is to unpivot data in a table. For example
let us consider a table that stores sales data in a wide format with columns for each month. By using cross apply with a table-valued function
you can unpivot the data into a long format with a row for each month.
Here is an example of how to use cross apply to unpivot data in SQL:
```
SELECT sales_id
month
amount
FROM sales
CROSS APPLY (VALUES ('January'
January)
('February'
February)
('March'
March)) AS Months(month
amount)
```
In this query
we are applying the table-valued function that returns the months and corresponding amounts for each row in the sales table. This will result in a new row for each month with the corresponding sales amount.
Cross apply can also be used to filter data based on a subquery. For example
you can apply a subquery to each row of a table and return only the rows that meet a specific condition.
Overall
cross apply is a powerful tool in SQL that allows you to perform complex calculations
transformations
and filtering operations on your data. By using cross apply strategically in your queries
you can achieve more efficient and flexible data manipulation.
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