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Functions - SQL Server

 Functions

In SQL Server, a function is a pre-written piece of code that performs a specific task and returns a value. Functions can be used to encapsulate complex calculations, to improve performance by reducing network traffic, and to enforce security by restricting direct access to database tables.

There are two types of functions in SQL Server:

1.     Scalar Functions: These functions return a single value based on input parameters. Examples include LEN, which returns the length of a string, and SUM, which returns the sum of a set of values.

2.     Table-Valued Functions: These functions return a table or a set of rows based on input parameters. Examples include fnSplit, which splits a string into rows based on a delimiter, and fnGetSalesByMonth, which returns sales data grouped by month.

To create a function in SQL Server, you can use the CREATE FUNCTION statement. Here's an example of creating a scalar function:

CREATE FUNCTION GetTotalOrders

(

    @CustomerID int

)

RETURNS int

AS

BEGIN

    DECLARE @TotalOrders int;

    SELECT @TotalOrders = COUNT(*) FROM Orders WHERE CustomerID = @CustomerID;

    RETURN @TotalOrders;

END

In this example, we're creating a scalar function called GetTotalOrders that takes an input parameter called @CustomerID and returns an integer value. The function uses a SELECT statement to count the number of orders for the specified customer ID and stores the result in a variable called @TotalOrders. Finally, the function returns the value of @TotalOrders.

Here's an example of creating a table-valued function:

CREATE FUNCTION fnGetSalesByMonth

(

    @Year int

)

RETURNS TABLE

AS

RETURN

(

    SELECT MONTH(OrderDate) AS Month, SUM(TotalAmount) AS TotalSales

    FROM Orders

    WHERE YEAR(OrderDate) = @Year

    GROUP BY MONTH(OrderDate)

)

 

In this example, we're creating a table-valued function called fnGetSalesByMonth that takes an input parameter called @Year and returns a table with two columns: Month and TotalSales. The function uses a SELECT statement to retrieve sales data for the specified year, grouping the data by month and calculating the total sales for each month.

Once you've created the function, you can call it like any other function. For example:

SELECT dbo.GetTotalOrders(12345);

 In this example, we're calling the GetTotalOrders function with an input parameter of 12345. The function will return the total number of orders for the

specified customer ID.

Reference :

            https://learn.microsoft.com/en-us/sql/t-sql/statements/create-function-transact-sql?view=sql-server-ver16

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