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Retrieving Data from a Function
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You retrieve data from a function by using a SELECT statement. Functions can be used in any of the following:
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A SELECT list A WHERE clause An expression A CHECK or DEFAULT constraint A FROM clause with the CROSS/OUTER APPLY function
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How a function is used can have a dramatic impact on the performance of the queries that you execute. A function in the SELECT list is used to calculate an aggregate or perform a computation on one or more columns of the tables in the FROM clause. A function in the WHERE clause is used to restrict a result set based on the results of the function.
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Programming Microsoft SQL Server with T-SQL User-Defined Stored Procedures, Functions, Triggers, and Views
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You should not create queries that use a function in the WHERE clause because the function would have to execute for each potential row returned from the results of the FROM clause. If you constructed a SELECT statement with a join between TableA and TableB that produced 100 rows matching the join, a function in the WHERE clause would be executed 100 times. If the result of the join produced 10,000 rows, the function would be executed 10,000 times. While you would not be reusing code, if you merged the code within the function into your SELECT statement, you would eliminate all the repetitive queries being executed.
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Functions can be nested inside each other so long as the return value of an inner function matches the input parameter of the outer function. For example, a common string-parsing routine might contain code as follows: DATALENGTH(POS(CHARINDEX(REPLACE(...)))). Functions in the CHECK and DEFAULT constraints are used to extend the static computations available. For example, if you want to validate the area code for a phone number against a list of area codes stored within a table, you can use a function to perform the validation that would not typically be possible because a CHECK constraint doesn t accept a SELECT statement.
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An inline table-valued function behaves like and is interchangeable with a view.
Quick check
1. What are the three types of functions that you can create 2. What are the required elements of a function
Quick check answers
1. You can create a scalar function, which returns a single value, an inline table-valued
function, which contains a single SELECT statement and is treated the same as a view, and a multi-statement table-valued function, which returns a table.
2. Every function ends with a RETURN statement. Scalar functions include the value
to be returned immediately following the RETURN statement. Inline table-valued functions include the SELECT statement for the result set to return immediately following the RETURN statement. Multi-statement table-valued functions just terminate with a RETURN. With the exception of inline table-valued functions, the entire function body is required to be enclosed in a BEGIN. . .END block.
Lesson 2: User-Defined Functions
Pr actice
creating Functions
In this practice, you create and use three different types of functions.
e xercise 1
Create a Scalar Function
In this exercise, you create and use a scalar function to return the current inventory of a product.
Open a new query window, type and execute the following code to create the function:
CREATE FUNCTION Production.udf_GetProductInventory (@ProductID INT) RETURNS INT AS --There are several locations for a product during each stage -- of manufacturing. We only want finished goods.
--A product can also be stored on multiple shelves and bins, so -- we need to sum the quantities. BEGIN DECLARE @Inventory INT
SELECT @Inventory = SUM(a.Quantity) FROM Production.ProductInventory a INNER JOIN Production.Location b ON a.LocationID = b.LocationID WHERE a.ProductID = @ProductID AND b.Name IN ('Miscellaneous Storage','Finished Goods Storage')
IF (@Inventory IS NULL) BEGIN SET @Inventory = 0 END RETURN @Inventory END GO
In the existing query window, type, highlight, and execute the following code to return results from the function:
SELECT Production.udf_GetProductInventory (325) GO
In the existing query window, type, highlight, and execute the following code to verify the results:
SELECT SUM(a.Quantity) FROM Production.ProductInventory a INNER JOIN Production.Location b ON a.LocationID = b.LocationID WHERE a.ProductID = 325 AND b.Name IN ('Miscellaneous Storage','Finished Goods Storage') GO
Programming Microsoft SQL Server with T-SQL User-Defined Stored Procedures, Functions, Triggers, and Views
e xercise 2
Create an Inline Table-Valued Function
In this exercise, you create and use an inline table-valued function to return all orders that have not yet shipped.
Open a new query window, type, highlight, and execute the following code to create an unshipped order:
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